diff --git a/arangod/Aql/IndexNode.cpp b/arangod/Aql/IndexNode.cpp
index 95597ef503..bf02d098c5 100644
--- a/arangod/Aql/IndexNode.cpp
+++ b/arangod/Aql/IndexNode.cpp
@@ -70,13 +70,13 @@ IndexNode::IndexNode(ExecutionPlan* plan, arangodb::velocypack::Slice const& bas
     std::string msg("collection '");
     msg.append(base.get("collection").copyString());
     msg.append("' not found");
-    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, msg);
+    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_ARANGO_COLLECTION_NOT_FOUND, msg);
   }
 
   VPackSlice indexes = base.get("indexes");
 
   if (!indexes.isArray()) {
-    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "\"indexes\" attribute should be an array");
+    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_BAD_PARAMETER, "\"indexes\" attribute should be an array");
   }
 
   _indexes.reserve(indexes.length());
@@ -89,7 +89,7 @@ IndexNode::IndexNode(ExecutionPlan* plan, arangodb::velocypack::Slice const& bas
 
   VPackSlice condition = base.get("condition");
   if (!condition.isObject()) {
-    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "\"condition\" attribute should be an object");
+    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_BAD_PARAMETER, "\"condition\" attribute should be an object");
   }
 
   _condition = Condition::fromVPack(plan, condition);
diff --git a/arangod/Aql/Query.cpp b/arangod/Aql/Query.cpp
index 19501ebbcd..3d61609923 100644
--- a/arangod/Aql/Query.cpp
+++ b/arangod/Aql/Query.cpp
@@ -470,7 +470,7 @@ ExecutionPlan* Query::prepare() {
     enterState(QueryExecutionState::ValueType::PLAN_INSTANTIATION);
     plan.reset(ExecutionPlan::instantiateFromAst(_ast.get()));
 
-    if (plan.get() == nullptr) {
+    if (plan == nullptr) {
       // oops
       THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "failed to create query execution engine");
     }
@@ -507,7 +507,7 @@ ExecutionPlan* Query::prepare() {
 
     // we have an execution plan in VelocyPack format
     plan.reset(ExecutionPlan::instantiateFromVelocyPack(_ast.get(), _queryBuilder->slice()));
-    if (plan.get() == nullptr) {
+    if (plan == nullptr) {
       // oops
       THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "could not create plan from vpack");
     }
@@ -980,7 +980,7 @@ QueryResult Query::explain() {
 
     if (plan == nullptr) {
       // oops
-      return QueryResult(TRI_ERROR_INTERNAL);
+      return QueryResult(TRI_ERROR_INTERNAL, "unable to create plan from AST");
     }
 
     // Run the query optimizer:
@@ -1074,8 +1074,8 @@ void Query::enterContext() {
       _context = V8DealerFeature::DEALER->enterContext(_vocbase, false);
 
       if (_context == nullptr) {
-        THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL,
-                                       "cannot enter V8 context");
+        THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_RESOURCE_LIMIT,
+                                       "unable to enter V8 context for query execution");
       }
 
       // register transaction and resolver in context
diff --git a/arangod/Aql/V8Executor.cpp b/arangod/Aql/V8Executor.cpp
index 27d755e62e..0950d8a536 100644
--- a/arangod/Aql/V8Executor.cpp
+++ b/arangod/Aql/V8Executor.cpp
@@ -359,7 +359,7 @@ void V8Executor::HandleV8Error(v8::TryCatch& tryCatch,
       msg += " See log for details";
     }
     // we can't figure out what kind of error occurred and throw a generic error
-    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, msg);
+    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_QUERY_SCRIPT, msg);
   }
 
   if (result.IsEmpty()) {
@@ -373,7 +373,7 @@ void V8Executor::HandleV8Error(v8::TryCatch& tryCatch,
       msg += " See log for details";
     }
 
-    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, msg);
+    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_QUERY_SCRIPT, msg);
   }
 
   // if we get here, no exception has been raised
diff --git a/arangod/RestHandler/RestBatchHandler.cpp b/arangod/RestHandler/RestBatchHandler.cpp
index e669bc30d3..9feddb76d7 100644
--- a/arangod/RestHandler/RestBatchHandler.cpp
+++ b/arangod/RestHandler/RestBatchHandler.cpp
@@ -30,6 +30,7 @@
 #include "GeneralServer/RestHandlerFactory.h"
 #include "Logger/Logger.h"
 #include "Rest/HttpRequest.h"
+#include "Utils/ExecContext.h"
 
 using namespace arangodb;
 using namespace arangodb::basics;
@@ -212,19 +213,14 @@ RestStatus RestBatchHandler::executeHttp() {
     {
       // ignore any errors here, will be handled later by inspecting the response
       try {
+        ExecContext* old = ExecContext::CURRENT;
+        ExecContext::CURRENT = nullptr;
+
+        TRI_DEFER(ExecContext::CURRENT = old);
         handler->syncRunEngine();
       } catch (...) {
       }
 
-#if 0
-      int result = TRI_ERROR_NO_ERROR; 
-      if (result != TRI_ERROR_NO_ERROR) {
-        generateError(rest::ResponseCode::BAD, TRI_ERROR_INTERNAL,
-                      "executing a handler for batch part failed");
-
-        return RestStatus::FAIL;
-      }
-#endif
       HttpResponse* partResponse =
           dynamic_cast<HttpResponse*>(handler->response());
 
diff --git a/arangod/RestHandler/RestExplainHandler.cpp b/arangod/RestHandler/RestExplainHandler.cpp
index c11c8f71ed..016919a1d0 100644
--- a/arangod/RestHandler/RestExplainHandler.cpp
+++ b/arangod/RestHandler/RestExplainHandler.cpp
@@ -82,7 +82,7 @@ void RestExplainHandler::explainQuery() {
     generateError(rest::ResponseCode::BAD, TRI_ERROR_BAD_PARAMETER, msg);
   };
 
-  if (!body.isObject() || body.length() < 1 || body.length() > 3) {
+  if (!body.isObject()) {
     badParamError(
         "expected usage: AQL_EXPLAIN(<queryString>, <bindVars>, "
         "<options>)");
diff --git a/js/server/modules/@arangodb/cluster.js b/js/server/modules/@arangodb/cluster.js
index 6f4d506c98..a016715566 100644
--- a/js/server/modules/@arangodb/cluster.js
+++ b/js/server/modules/@arangodb/cluster.js
@@ -597,7 +597,7 @@ function synchronizeOneShard (database, shard, planId, leader) {
             try {
               ok = addShardFollower(ep, database, shard, lockJobId);
             } catch (err4) {
-              db._drop(shard);
+              db._drop(shard, {isSystem: true });
               throw err4;
             }
           } catch (err3) {
@@ -1001,7 +1001,7 @@ function executePlanForCollections(plannedCollections) {
               collections[collection].planId);
 
             try {
-              db._drop(collection, {timeout:1.0});
+              db._drop(collection, {timeout:1.0, isSystem: true});
               console.topic('heartbeat=debug', "dropping local shard '%s/%s' of '%s/%s => SUCCESS",
                     database,
                     collection,