I've been experiencing this since ever but I've always postponed any investigation to when I had more time. Having noticed that I never got around to do anything about it, I thought I might as well involve the SLIME community.
About one out of five times I start slime, I get:
;; Swank started at port: 49576. 49576 * ;; swank:close-connection: End-of-File on #<Stream for descriptor 9>
Through the inferior lisp buffer I can verify that the Lisp is not dead; I can interact without problems. It seems like it's just SLIME that fails to notice that.
If I do another M-x slime, Emacs asks me if I want to create another inferior lisp. Answering "no" the Lisp session is rescued and everything proceeds as normal.
Has anyone else seen something like that?
Just for the record, I'm using CMUCL 19d on FreeBSD.