When the typeout frame is active, autodoc output is sent to the frame using a call to SLIME-TYPEOUT-MESSAGE in SLIME-AUTODOC-MESSAGE. In turn, SLIME-TYPEOUT-MESSAGE applies FORMAT to the string to display it. This causes a fair number of errors when the string contains the % character and eventually autodoc-mode shuts off.
I've attached a patch to fix it against the latest version. All it does is call REPLACE-REGEXP-IN-STRING before the string is sent to SLIME-TYPEOUT-MESSAGE.
I'm not convinced this solution is at all elegant, but I'm not familiar with the architecture of Slime enough to know what a better solution might be. It's a hack since it's not terribly robust: right now no format arguments are passed to SLIME-TYPEOUT-MESSAGE but if they are, then the hack provided (likely) won't work.
Anyway, this patch solves my immediate problem and I thought it was a point worth bringing up.
And am I one of the only people to use a typeout frame? :)