What version of the Websphere MQ adapter are you using? Did you apply the service packs and fixes as suggested by WM support? The about page in my adapter shows version 3.0, build 113, patch SP1. There could be something newer, so be sure to check with support.
My simple listen example uses separate services for the message processing callback service and the exception handling callback service with no problems.
If you use the identical service for these two parameters, it would be called both when a normal message is received and when an exception occurs. The only way to tell whether your are processing a normal message or an exception message is by examining the message content for null values in key fields (like messageBody).
Mark
#Universal-Messaging-Broker#broker#webMethods#Integration-Server-and-ESB