I just had an "interesting" bug. It popped up *two* debuggers - one in Tweak, and one in Morphic.
The first was a genuine keyNotFound error in a setupCostume method. This prevented the player from opening fully. However, a geometryChanged event was already triggered, and the handler for that accessed the bounds of the player's container which was still nil, since the opening did not succeed. Thus, the second error was a "MNU: UndefinedObject>>bounds".
The odd thing is that the second error did not just pop up another Tweak debugger, but (through the ErrorRecursion logic) brought up a Morphic debugger on the Tweak debugger (rather than on the guilty context). I had to manually print the stack of aContext in there to find out what was actually going on.
I think that the error recursion logic would have to be made more selective, because a second error that was *not* caused by opening the Tweak debugger should indeed bring up another Tweak debugger. Only if opening the debugger itself fails than we should fall back to whatever other means we have.