diff --git a/java/com/google/gerrit/server/restapi/account/QueryAccounts.java b/java/com/google/gerrit/server/restapi/account/QueryAccounts.java
index a40cdd0..f9e753c 100644
--- a/java/com/google/gerrit/server/restapi/account/QueryAccounts.java
+++ b/java/com/google/gerrit/server/restapi/account/QueryAccounts.java
@@ -43,6 +43,7 @@
 import com.google.gerrit.server.query.account.AccountQueryBuilder;
 import com.google.gerrit.server.query.account.AccountQueryProcessor;
 import com.google.inject.Inject;
+import com.google.inject.Provider;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.LinkedHashMap;
@@ -58,12 +59,13 @@
   private final PermissionBackend permissionBackend;
   private final AccountLoader.Factory accountLoaderFactory;
   private final AccountQueryBuilder queryBuilder;
-  private final AccountQueryProcessor queryProcessor;
+  private final Provider<AccountQueryProcessor> queryProcessorProvider;
   private final boolean suggestConfig;
   private final int suggestFrom;
 
   private AccountLoader accountLoader;
   private boolean suggest;
+  private Integer limit;
   private int suggestLimit = 10;
   private String query;
   private Integer start;
@@ -80,7 +82,7 @@
       metaVar = "CNT",
       usage = "maximum number of users to return")
   public void setLimit(int n) {
-    queryProcessor.setUserProvidedLimit(n);
+    this.limit = n;
 
     if (n < 0) {
       suggestLimit = 10;
@@ -124,12 +126,12 @@
       PermissionBackend permissionBackend,
       AccountLoader.Factory accountLoaderFactory,
       AccountQueryBuilder queryBuilder,
-      AccountQueryProcessor queryProcessor,
+      Provider<AccountQueryProcessor> queryProcessorProvider,
       @GerritServerConfig Config cfg) {
     this.permissionBackend = permissionBackend;
     this.accountLoaderFactory = accountLoaderFactory;
     this.queryBuilder = queryBuilder;
-    this.queryProcessor = queryProcessor;
+    this.queryProcessorProvider = queryProcessorProvider;
     this.suggestFrom = cfg.getInt("suggest", null, "from", 0);
     this.options = EnumSet.noneOf(ListAccountsOption.class);
 
@@ -186,10 +188,15 @@
     }
     accountLoader = accountLoaderFactory.create(fillOptions);
 
+    AccountQueryProcessor queryProcessor = queryProcessorProvider.get();
     if (queryProcessor.isDisabled()) {
       throw new MethodNotAllowedException("query disabled");
     }
 
+    if (limit != null) {
+      queryProcessor.setUserProvidedLimit(limit);
+    }
+
     if (start != null) {
       queryProcessor.setStart(start);
     }
diff --git a/java/com/google/gerrit/server/restapi/group/QueryGroups.java b/java/com/google/gerrit/server/restapi/group/QueryGroups.java
index 816fcf6..a233111 100644
--- a/java/com/google/gerrit/server/restapi/group/QueryGroups.java
+++ b/java/com/google/gerrit/server/restapi/group/QueryGroups.java
@@ -32,6 +32,7 @@
 import com.google.gerrit.server.query.group.GroupQueryBuilder;
 import com.google.gerrit.server.query.group.GroupQueryProcessor;
 import com.google.inject.Inject;
+import com.google.inject.Provider;
 import java.util.ArrayList;
 import java.util.EnumSet;
 import java.util.List;
@@ -39,7 +40,7 @@
 
 public class QueryGroups implements RestReadView<TopLevelResource> {
   private final GroupQueryBuilder queryBuilder;
-  private final GroupQueryProcessor queryProcessor;
+  private final Provider<GroupQueryProcessor> queryProcessorProvider;
   private final GroupJson json;
 
   private String query;
@@ -88,9 +89,11 @@
 
   @Inject
   protected QueryGroups(
-      GroupQueryBuilder queryBuilder, GroupQueryProcessor queryProcessor, GroupJson json) {
+      GroupQueryBuilder queryBuilder,
+      Provider<GroupQueryProcessor> queryProcessorProvider,
+      GroupJson json) {
     this.queryBuilder = queryBuilder;
-    this.queryProcessor = queryProcessor;
+    this.queryProcessorProvider = queryProcessorProvider;
     this.json = json;
   }
 
@@ -101,6 +104,8 @@
       throw new BadRequestException("missing query field");
     }
 
+    GroupQueryProcessor queryProcessor = queryProcessorProvider.get();
+
     if (queryProcessor.isDisabled()) {
       throw new MethodNotAllowedException("query disabled");
     }
diff --git a/java/com/google/gerrit/server/restapi/project/DeleteBranch.java b/java/com/google/gerrit/server/restapi/project/DeleteBranch.java
index 2a20283..6248a61 100644
--- a/java/com/google/gerrit/server/restapi/project/DeleteBranch.java
+++ b/java/com/google/gerrit/server/restapi/project/DeleteBranch.java
@@ -17,6 +17,7 @@
 import static com.google.gerrit.entities.RefNames.isConfigRef;
 import static org.eclipse.jgit.lib.Constants.R_HEADS;
 
+import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.extensions.common.Input;
 import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
@@ -46,7 +47,9 @@
   @Override
   public Response<?> apply(BranchResource rsrc, Input input)
       throws RestApiException, IOException, PermissionBackendException {
-    if (isConfigRef(rsrc.getBranchKey().branch())) {
+    if (RefNames.HEAD.equals(rsrc.getBranchKey().branch())) {
+      throw new MethodNotAllowedException("not allowed to delete HEAD");
+    } else if (isConfigRef(rsrc.getBranchKey().branch())) {
       // Never allow to delete the meta config branch.
       throw new MethodNotAllowedException(
           "not allowed to delete branch " + rsrc.getBranchKey().branch());
diff --git a/java/com/google/gerrit/server/restapi/project/DeleteBranches.java b/java/com/google/gerrit/server/restapi/project/DeleteBranches.java
index d429655..ca5962e 100644
--- a/java/com/google/gerrit/server/restapi/project/DeleteBranches.java
+++ b/java/com/google/gerrit/server/restapi/project/DeleteBranches.java
@@ -17,8 +17,10 @@
 import static org.eclipse.jgit.lib.Constants.R_HEADS;
 
 import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.extensions.api.projects.DeleteBranchesInput;
 import com.google.gerrit.extensions.restapi.BadRequestException;
+import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
 import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
@@ -43,6 +45,14 @@
     if (input == null || input.branches == null || input.branches.isEmpty()) {
       throw new BadRequestException("branches must be specified");
     }
+
+    if (input.branches.contains(RefNames.HEAD)) {
+      throw new MethodNotAllowedException("not allowed to delete HEAD");
+    } else if (input.branches.stream().anyMatch(RefNames::isConfigRef)) {
+      // Never allow to delete the meta config branch.
+      throw new MethodNotAllowedException("not allowed to delete branch " + RefNames.REFS_CONFIG);
+    }
+
     deleteRef.deleteMultipleRefs(
         project.getProjectState(), ImmutableSet.copyOf(input.branches), R_HEADS);
     return Response.none();
diff --git a/java/com/google/gerrit/server/submit/MergeSuperSet.java b/java/com/google/gerrit/server/submit/MergeSuperSet.java
index c690e0b..bcebc7f 100644
--- a/java/com/google/gerrit/server/submit/MergeSuperSet.java
+++ b/java/com/google/gerrit/server/submit/MergeSuperSet.java
@@ -99,7 +99,11 @@
         closeOrm = true;
       }
       List<ChangeData> cds = queryProvider.get().byLegacyChangeId(change.getId());
-      checkState(cds.size() == 1, "Expected exactly one ChangeData, got " + cds.size());
+      checkState(
+          cds.size() == 1,
+          "Expected exactly one ChangeData for change ID %s, got %s",
+          change.getId(),
+          cds.size());
       ChangeData cd = Iterables.getFirst(cds, null);
 
       boolean visible = false;
diff --git a/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchIT.java b/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchIT.java
index 4f1be30..5636014 100644
--- a/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchIT.java
@@ -33,6 +33,7 @@
 import com.google.gerrit.extensions.api.projects.BranchInput;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.IdString;
+import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.inject.Inject;
@@ -172,6 +173,24 @@
     assertThat(thrown).hasMessageThat().contains("Not allowed to delete group branch.");
   }
 
+  @Test
+  public void cannotDeleteRefsMetaConfig() throws Exception {
+    MethodNotAllowedException thrown =
+        assertThrows(
+            MethodNotAllowedException.class,
+            () -> branch(BranchNameKey.create(allUsers, RefNames.REFS_CONFIG)).delete());
+    assertThat(thrown).hasMessageThat().contains("not allowed to delete branch refs/meta/config");
+  }
+
+  @Test
+  public void cannotDeleteHead() throws Exception {
+    MethodNotAllowedException thrown =
+        assertThrows(
+            MethodNotAllowedException.class,
+            () -> branch(BranchNameKey.create(allUsers, RefNames.HEAD)).delete());
+    assertThat(thrown).hasMessageThat().contains("not allowed to delete HEAD");
+  }
+
   private void blockForcePush() throws Exception {
     projectOperations
         .project(project)
diff --git a/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchesIT.java b/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchesIT.java
index 3cdeafd..ad90109 100644
--- a/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchesIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/project/DeleteBranchesIT.java
@@ -35,6 +35,7 @@
 import com.google.gerrit.extensions.api.projects.ProjectApi;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
+import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.inject.Inject;
 import java.util.HashMap;
@@ -156,6 +157,26 @@
     assertThat(thrown).hasMessageThat().contains("branches must be specified");
   }
 
+  @Test
+  public void cannotDeleteRefsMetaConfig() throws Exception {
+    DeleteBranchesInput input = new DeleteBranchesInput();
+    input.branches = Lists.newArrayList();
+    input.branches.add(RefNames.REFS_CONFIG);
+    MethodNotAllowedException thrown =
+        assertThrows(MethodNotAllowedException.class, () -> project().deleteBranches(input));
+    assertThat(thrown).hasMessageThat().contains("not allowed to delete branch refs/meta/config");
+  }
+
+  @Test
+  public void cannotDeleteHead() throws Exception {
+    DeleteBranchesInput input = new DeleteBranchesInput();
+    input.branches = Lists.newArrayList();
+    input.branches.add(RefNames.HEAD);
+    MethodNotAllowedException thrown =
+        assertThrows(MethodNotAllowedException.class, () -> project().deleteBranches(input));
+    assertThat(thrown).hasMessageThat().contains("not allowed to delete HEAD");
+  }
+
   private String errorMessageForBranches(List<String> branches) {
     StringBuilder message = new StringBuilder();
     for (String branch : branches) {