Index: src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	(revision 899132)
+++ src/main/java/org/apache/jackrabbit/core/state/SharedItemStateManager.java	(working copy)
@@ -572,6 +572,8 @@
                     checkReferentialIntegrity();
                 }
 
+                checkAddedChildNodes();
+
                 /**
                  * prepare the events. this needs to be after the referential
                  * integrity check, since another transaction could have modified
@@ -957,6 +959,40 @@
         }
 
         /**
+         * 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 (Iterator iter = local.addedStates(); iter.hasNext();) {
+                checkAddedChildNode((ItemState) iter.next());
+            }
+            for (Iterator iter = local.modifiedStates(); iter.hasNext();) {
+                checkAddedChildNode((ItemState) iter.next());
+            }
+        }
+
+        private void checkAddedChildNode(ItemState state) throws ItemStateException {
+            if (state.isNode()) {
+                NodeState node = (NodeState) state;
+
+                for (Iterator iter = node.getAddedChildNodeEntries().iterator();
+                     iter.hasNext();) {
+                    ChildNodeEntry cne =
+                        (ChildNodeEntry) iter.next();
+                    NodeId id = cne.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>
Index: src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java	(revision 899132)
+++ src/main/java/org/apache/jackrabbit/core/observation/EventStateCollection.java	(working copy)
@@ -364,6 +364,11 @@
                 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 mixins = parent.getMixinTypeNames();
                 Path path = getPath(n.getNodeId(), hmgr);
@@ -377,6 +382,11 @@
             } 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 mixins = n.getMixinTypeNames();
                 Path path = getPath(state.getId(), hmgr);
@@ -474,7 +484,12 @@
         } 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/ItemImpl.java
===================================================================
--- src/main/java/org/apache/jackrabbit/core/ItemImpl.java	(revision 899132)
+++ src/main/java/org/apache/jackrabbit/core/ItemImpl.java	(working copy)
@@ -68,6 +68,7 @@
 import javax.jcr.version.VersionException;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.ConcurrentModificationException;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Set;
@@ -926,7 +927,16 @@
              * build list of transient (i.e. new & modified) states that
              * should be persisted
              */
-            Collection dirty = getTransientStates();
+            Collection 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;