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.