Don - your fix should have solved the problem. There are (actually were, if my patch is accepted and applied) several problems with user-join.tcl, the worst being that the code was using ad_complain outside of ad_page_contract's -validate block which was doing nothing at all. It should have used ad_return_complaint and then return(ed) to the caller, and my patch handles this issue. There were checks for 'closed' groups and segments but the code wasn't handling returning any errors as I stated above.
Another issue was that user-join was not requiring a registered user so the unregistered user could have been added to any group with a non-closed join_policy. I fixed this be requiring a registered user for the request.
I also added (slightly) more descriptive text when an error is reported with ad_return_complaint, and I added a "return to previous page" link if return_url is contained in the query vars.
Lastly, I cleaned up the on_error block by checking the error message for a unique constraint violation that would indicate that the user is requesting membership in a group to which she already belongs. This condition is displayed along with the "return to previous..." link.
I think this hole is quite serious. That's why I took the time to fix it and generate a patch against today's 4.6 CVS tip. I hope this can be applied soon, and that site owners are aware that this hole exists.