Without naming names (which I couldn't anyway because I've forgotten exactly
who it was), one the most amusing bug reports we received at Lucid wrt Lucid
Common Lisp was that "GC runs too fast". Someone had a C program they
converted to Lisp and it ran twice as fast. Since they "knew" the minimum
time needed to allocate and deallocate their data and we were running faster
than that, they assumed we had buggy code.
Granted, it might have been rare, but it was a real example.