Thanks - I found those two - Jochen below narrowed it down.

Thanks - I must have been too tired to figure that one out.

This break in CWnd::WindowProc randomly appears when running my application(originally released in 2010 and runs 24/7 in many places) in debug. In release mode it crashes. The message type is always 874(x36A). The only true call backs I have are for custom draw list boxes and typically when I have an error in those routines MFC catches and breaks at that point. Obviously that are countless other messages processed. I am trying to determine A: what is message 874? and B: what circumstances cause it to be posted (though A may answer B)?