Hallo,
I have been toying with gambit and afl[1]. It's really a remarkable tool. It has found some code that is not well-behaved. For instance, this hangs gsi:
( six.!x ( top ))
There are more hangs and a few crashes. It has only been running for one day, I plan to check on it again on Monday.
[1] http://lcamtuf.coredump.cx/afl/
Cheers,