Spot the bug! (From the SBCL debugger internals)
;; pointer (and (logand val 1) ;; Check that the pointer is valid XXX Could do a better ;; job FIXME: eg by calling out to an is_valid_pointer ;; routine in the C runtime support code )
The bug caused the generation of invalid lispobjs under some very rare circumstances (and only on 64-bit SBCL), leading to failing GC assertions much further down the road. Even after isolating the problem to one function (which took a while) I couldn't spot the actual bug without adding copious amounts of debug output. ETOOMUCHC.