Try doing the following in a Slime REPL buffer, in SBCL:

Type (/ 1 0), entering SLDB

Type E, get prompted for a form to eval

Type (aref nil 0), entering a recursive SLDB

Type 0, to return to the outer SLDB.

Fails: Swank tries to evaluate *sldb-stack-top* but it is unbound.

-- Dan