Many Lisp implementations (including CCL, CLISP and SBCL) have a command-line option that causes unhandled errors to terminate the Lisp process instead of invoking the debugger. This option is useful for programs that don't have irreplaceable valuable state in the heap, and that have users whose reaction upon seeing a debug REPL would be "How do I get out of this!?". Also, a few days ago someone on IRC mentioned having the problem that "if there is an error in the expression" the server thread "gets locked in the ABCL repl". I think his or her use-case might also benefit from a debugger-disabling switch. Happy Thanksgiving!