Earlier quoted context omitted.
This seems to be a recurring theme with Lisp implementations. Reddit switched from CMUCL to Python (threading); Vendetta Online switched from SBCL to Erlang (GC/memory leaks). Other people have hit major snags e.g. with Haskell/GHC runtime bugs (Wager Labs; switched to Erlang). Use a massively complex runtime, hit bugs that can't be fixed (in a start-up timeframe, anyhow)?
Reddit switched from CMUCL to Python (threading) Source? With the bashing Python is getting because of the GIL, it surprises me that someone would switch to Python from elsewhere for any reasons related to threading.
Straight from PG:
"Incidentally, the last straw, I've been told, was some bug in CMUCL threads that kept making the system crash."
http://groups.google.com/group/comp.lang.lisp/msg/574d19d5e7...
From Reddit: http://blog.reddit.com/2005/12/on-lisp.html