Ahoy,
In CMUCL and non-threads SBCL we default to using a 'SIGIO' signal handler to read and execute requests from Emacs. This means that Lisp is interrupted to immediately serve requests as soon as Emacs sends them. It is convenient, but it has potential to cause unpredictable problems.
For me this has been working perfectly, but I've been hearing reports that some people are having stability problems. If you're using CMUCL or non-threads SBCL and are finding Lisp unstable under SLIME, please try adding this to your ~/.swank.lisp:
(setq swank:*communication-style* :fd-handler)
That way we use the select()-loop instead of signals. It ought to be very safe, though Lisp will only process requests from Emacs when it's otherwise idle.
If this solves a problem for you then please let us know. Currently I'm not sure whether SIGIO or FD-HANDLER makes the better default.
NB: This stuff is covered in the manual under Lisp-Side configuration.
Cheers, Luke