when `debug', abort when lisp error during loadup loadup.el: When featurep `debug-xemacs' (configure --with-debug), set debug-on-error, so that we get an exit-to-debugger/assertion failure upon Lisp error during loadup. Unset before dumping. cmdloop.c: During really-early-error-handler, exit to the debugger and abort if an error occurs and lisp.h: extern Vdebug_on_error.