I notice that on one of my lists, a moderator had been using the "Add (address) to a sender filter" checkbox on a lot of pending moderator requests, so that a substantial list of automatic-discard addresses had been built *without* ever using the Sender Filters admin page.

When I went to the Sender Filters page myself and tried to change something else, the Submit blew up with

TypeError: not enough arguments for format string

in htmlformat.py . I googled this and on a couple of Python forums I found the suggestion than an un-escaped percent sign had found its way into a template. My templates are untouched but Mailman builds a lot of documents on the fly, so I scanned the pre-submit Sender Filters page source for percents and lo, there were a couple in the list of auto-discard email addresses. On a whim I removed them and re-Submitted. Instant success.

I am too swamped to hack the Mailman code right now but if someone wants to take a look, it may be that there is something called addError that is passing un-escaped text. We might want to come in after any template substitution and escape what's left. Or something.