Exception thrown without a stack frame

I'm getting exceptions thrown without a stackframe. I understand what this means but not how it can happen given the following: 1. *every* frontend script on the site is wrapped in a try/catch block 2. I have set an exception handler to dump these so-called uncaught exceptions in the error log I use APC on the site/server in question but it doesn't matter whether I actually load or use APC as to whether these uncaught exceptions occur. I can't seem to reproduce the problem from my own browser - although it's difficult to determine much from the log given the lack of information in any entry referring to 'exception thrown without a stack frame' - apparently the uncaught exception is not causing users to see broken or blank pages. The problem only occurs on the production server, I can't reproduce it on my test server no matter how hard I push with ab. I haven't updated php for quite a long time which is at version 5.1.1 has anyone ever experienced something like this? anyone have any clue as to what it might be. tia