break when encountering a hard or soft error caused by ACL2.
Major Section:  TRACE

General forms:
(break-on-error t)   ; installs a trace causing a continuable error (break)
                     ;   whenever a hard or soft error is invoked by ACL2.
(break-on-error)     ; same as above
(break-on-error nil) ; uninstall the above trace
(Break-on-error) is actually a macro that expands as follows.
(trace$ (illegal :entry (break))
        (error1  :entry (break)))
This trace should cause entry to the Lisp debugger (at least in most Lisps) whenever ACL2 calls its error routines.

Also see trace$.