Comment 5 for bug 1251495

To reproduce the error you need to enable topics for a list, create a topic and send a post that matches that topic.

You can do the enable/create by putting these two lines

topics_enabled = 1
topics = [('Match_all', '.', 'Topic matches anything for testing', False)]

in a file and running Mailman's bin/config_list like

bin/config_list -i path/to/file/with/lines list_name

Then, a post to that list should be delivered normally and contain a header 'X-Topics: Match_all', but the bug will cause the post to be shunted.