What could cause my NSException to be swallowed up?

I have some assertion code that raises an NSException on bad input.

Today, I messed up one of my methods and generated a condition which triggered the exception. However, unlike usual, XCode didn't stop on the exception raise. It kept going. It took me a LOT of NSLog calls to figure this out, because it also unwound the entire call stack so I couldn't figure out where it was dying开发者_StackOverflow社区.

I have a custom unhandled exception function defined (AND breakpointed), and everything was working until today (when I have no idea what changed).

What could cause XCode and/or the Objective-C runtime to swallow my NSExceptions whole and not tell me?

Interestingly, the code kept running just fine - went back out to the main run loop.

==EDIT==

This just happened to me again, 5 days later, in a different place. Beginning to suspect buggy behavior in Xcode 4 while debugging. Watch this space.