Index: src/main/java/org/apache/jackrabbit/core/ItemImpl.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/ItemImpl.java	(revision 898289)
+++ src/main/java/org/apache/jackrabbit/core/ItemImpl.java	(working copy)
@@ -18,6 +18,7 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.ConcurrentModificationException;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -481,7 +482,7 @@
                     String msg = itemMgr.safeGetJCRPath(id)
                                 + ": mandatory child node " + cnd.getName()
                                 + " does not exist";
-                    if (!nodeState.hasChildNodeEntry(cnd.getName())) {                      
+                    if (!nodeState.hasChildNodeEntry(cnd.getName())) {
                         log.debug(msg);
                         throw new ConstraintViolationException(msg);
                     } else {
@@ -975,7 +976,15 @@
              * build list of transient (i.e. new & modified) states that
              * should be persisted
              */
-            Collection<ItemState> dirty = getTransientStates();
+            Collection<ItemState> dirty;
+            try {
+                dirty = getTransientStates();
+            } catch (ConcurrentModificationException e) {
+                String msg = "Concurrent modification; session is closed";
+                log.error(msg, e);
+                session.logout();
+                throw e;
+            }
             if (dirty.size() == 0) {
                 // no transient items, nothing to do here
                 return;
Index: src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java	(revision 898289)
+++ src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java	(working copy)
@@ -404,6 +404,11 @@
                 NodeId parentId = n.getParentId();
                 // the parent of an added item is always modified or new
                 NodeState parent = (NodeState) changes.get(parentId);
+                if (parent == null) {
+                    String msg = "Parent " + parentId + " must be changed as well.";
+                    log.error(msg);
+                    throw new ItemStateException(msg);
+                }
                 NodeTypeImpl nodeType = getNodeType(parent, session);
                 Set<Name> mixins = parent.getMixinTypeNames();
                 Path path = getPath(n.getNodeId(), hmgr);
@@ -420,6 +425,11 @@
             } else {
                 // property created / set
                 NodeState n = (NodeState) changes.get(state.getParentId());
+                if (n == null) {
+                    String msg = "Node " + state.getParentId() + " must be changed as well.";
+                    log.error(msg);
+                    throw new ItemStateException(msg);
+                }
                 NodeTypeImpl nodeType = getNodeType(n, session);
                 Set<Name> mixins = n.getMixinTypeNames();
                 Path path = getPath(state.getId(), hmgr);
@@ -625,7 +635,12 @@
         } catch (Exception e) {
             // also catch eventual runtime exceptions here
             // should never happen actually
-            String msg = "Item " + node.getNodeId() + " has unknown node type: " + node.getNodeTypeName();
+            String msg;
+            if (node == null) {
+                msg = "Node state is null";
+            } else {
+                msg = "Item " + node.getNodeId() + " has unknown node type: " + node.getNodeTypeName();
+            }
             log.error(msg);
             throw new ItemStateException(msg, e);
         }
Index: src/main/java/org/apache/jackrabbit/core/state/SessionItemStateManager.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/state/SessionItemStateManager.java	(revision 898289)
+++ src/main/java/org/apache/jackrabbit/core/state/SessionItemStateManager.java	(working copy)
@@ -41,9 +41,6 @@
 import org.apache.jackrabbit.core.util.Dumpable;
 import org.apache.jackrabbit.spi.Name;
 import org.apache.jackrabbit.spi.QNodeDefinition;
-import org.apache.jackrabbit.spi.Path;
-import org.apache.jackrabbit.spi.PathFactory;
-import org.apache.jackrabbit.spi.commons.name.PathFactoryImpl;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -582,7 +579,7 @@
         // the nearest common ancestor of all transient states
         // must be either
         // a) a node state with STATUS_EXISTING_MODIFIED, or
-        // b) the parent node of a property state with STATUS_EXISTING_MODIFIED 
+        // b) the parent node of a property state with STATUS_EXISTING_MODIFIED
 
         // collect all candidates based on above criteria
         Collection<NodeId> candidateIds = new LinkedList<NodeId>();
Index: src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	(revision 898289)
+++ src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	(working copy)
@@ -574,6 +574,8 @@
                     checkReferentialIntegrity();
                 }
 
+                checkAddedChildNodes();
+
                 /**
                  * prepare the events. this needs to be after the referential
                  * integrity check, since another transaction could have modified
@@ -731,8 +733,8 @@
                 long t0 = System.currentTimeMillis();
                 persistMgr.store(shared);
                 succeeded = true;
-                long t1 = System.currentTimeMillis();
                 if (log.isDebugEnabled()) {
+                    long t1 = System.currentTimeMillis();
                     log.debug("persisting change log " + shared + " took " + (t1 - t0) + "ms");
                 }
             } finally {
@@ -968,6 +970,36 @@
         }
 
         /**
+         * Verify the added child nodes of the added or modified states exist.
+         * If they don't exist, most likely the problem is that the same session
+         * is used concurrently.
+         */
+        private void checkAddedChildNodes() throws ItemStateException {
+            for (ItemState state : local.addedStates()) {
+                checkAddedChildNode(state);
+            }
+            for (ItemState state : local.modifiedStates()) {
+                checkAddedChildNode(state);
+            }
+        }
+
+        private void checkAddedChildNode(ItemState state) throws ItemStateException {
+            if (state.isNode()) {
+                NodeState node = (NodeState) state;
+                for (ChildNodeEntry child : node.getAddedChildNodeEntries()) {
+                    NodeId id = child.getId();
+                    if (local.get(id) == null) {
+                        if (!cache.isCached(id) && !persistMgr.exists(id)) {
+                            String msg = "Trying to add a non-existing child node: " + id;
+                            log.debug(msg);
+                            throw new ItemStateException(msg);
+                        }
+                    }
+                }
+            }
+        }
+
+        /**
          * Verifies that
          * <ul>
          * <li>no referenceable nodes are deleted if they are still being referenced</li>