The difference between what Stan does and what ad_maybe_redirect_for_registration boils down to one thing, from my reading of the code:
a call to ad_script_abort.
This calls "ad_raise" to abort the script, which in turn interacts with the request processor which handles the ad_script_abort condition.
The error's probably being triggered in there under some conditions but I've not dug into the request processor all that deeply so don't know offhand what might be causing the problem.
As others have said, though, it comes after the ad_returnredirect so it won't affect your users.
Is it possible the "once every 2K or so times" corresponds to the frequency with which your users wander into such pages without being logged in?
