We have investigated this using our own test harness for user defined documents and the system works properly. If there is an invalid tag the document just displays blank value there, but it does not hang.
Quite possibly your problem is related to the specific installation or version of MS Word.
If a hang happens please describe exactly the symptoms - are there are any exceptions in the output of the Aware IM Server and Tomcat? Is MS Word loaded in the memory of the Aware IM Server (check with the Task Manager). If so, what happens if you manually kill the Word process from memory. Will the system come back to life after this?