This one is intermittent, but easy to repro.
Repro steps: 1- Load any program (even "blink" can cause this) 2-Modify program 3-Upload the program 4-If it works, goto step 2
Result: After a few tries (sometimes about 10), OSX itself will crash before the upload stage is completed, and you'll have to reboot the mac.
Note: This happened to me even when I inadvertently forgot to plug the board via USB.
Needless to say, this is quite a showstopper, as crashing a Unix-like system isn't exactly recommended...