SLIME has a custom Emacs-based debugger called SLDB. Conditions
signalled in the Lisp system invoke SLDB in Emacs by way of the
Lisp *DEBUGGER-HOOK*.
SLDB pops up a buffer when a condition is signalled. The buffer displays a description of the condition, a list of restarts, and a backtrace. Commands are offered for invoking restarts, examining the backtrace, and poking around in stack frames.