Hi Eliot:
I already had that problem once, but now I got it reproducible.
A message is send to the wrong object. It is send to self instead of the third parameter of a method.
I will send you an image with the test case privately.
The problem seems to be in the Stack interpreter and in Cog. But it does not happen in the old interpreter (tested with Mac VM 5.7.4.1).
Best regards Stefan
vm-dev@lists.squeakfoundation.org