It's possible to view old, public versions of abusefilters that are currently marked private, by putting the ID of a public filter in the URL.
For example, version 22545 of private filter 1008 on enwiki was accidentally marked public, but the mistake was soon caught.
If we try to view it the "normal" way:
then we get an error, as expected. But if we pretend we're looking at filter 1 instead:
then we are shown the filter pattern, as if it were public. This leaves no way to fix the mistake of accidentally un-ticking the "private" box.