Enwiki private filter 789 has had false positives on Wikipedia:Administrators' newsletter/Subscribe. An example filter log can be seen here.
Without discussing details of the private filter, the cause of this is that a condition searches the added_lines variable. Although the diff looks normal when viewed in the usual way (here), when you look at the filter log details, removed_lines contains all the old version with spacing, and added_lines contains all the new version without spacing. This applies to the delivery list and the header.
This effectively makes added_lines equivalent to new_wikitext.