Hi,
The following commit
2011-05-22 Helmut Eller heller@common-lisp.net [...] (slime-inspector-quit): Drop the unused kill-buffer arg from the lambda-list.
causes problem with closing inspector buffers using "q", because slime-popup-buffer-quit-function expects the callback (specified by variable with the same name) to accept kill-buffer-p argument.
P.S. Unfortunately, the snapshot of 2011-05-22 got into quicklisp collection. It would be probably useful to notify Zach Beane directly when this bug is fixed.