It's happened to me a few times:
The "Select Sound File" dialog box gets stuck when loading a sample in the Drums contraption. The "Open" and "Cancel" buttons don't do anything. The only way out is to force quit.
I believe that each time the MIDI clock was running (AM chasing sync), as was the case just now.
On OSX 10.6.8.
I don't know whether it will help, but you could also try pressing the escape key on the keyboard.
Tried it of course. Doesn't work either.