Error: RT::User::crit Unimplemented in RT::User

RT e-mailed the following error and the bounced message to me. Any idea
why this would happen? RT has been running fine for a couple of weeks

RT server error. The RT server which handled your email did not behave as
expected. It said:
System error

error: RT::User::crit Unimplemented in RT::User.
(/usr/local/rt3/lib/RT/ line 206)

context: …
277: }
279: # All errors returned from this routine will be in exception form.
280: local $SIG{‘DIE’} = sub {
281: rethrow_exception( $_[0] );
282: };
284: #
285: # $m is a dynamically scoped global containing this

code stack:

Our config:
FreeBSD 4.7 / rt3.0.0 / Sendmail 8.12.6p2 / mysql 3.23.52 (InnoDB tables)
Apache/1.3.27 (Unix) mod_ssl/2.8.11 OpenSSL/0.9.6g mod_fastcgi/2.2.12