This guy at work could not figure out why some errors in his application were being caught by his error handling template (as defined by the CFError tag) and some were not. The errors being caught were handled nicely. The ones that were seemingly not caught were dumped out on to the page as if no error handler was defined.
This is a common problem that I see with people's error handling template. The problem here is that the CFError template will NOT handle errors that it produces. If this were the case, it could launch into some crazy, infinite loop of error handling. By surrounding the code of the error handling template with CFTry / CFCatch tags, it became immediately apparent that his error handler had a bug in it.