When selecting a namespace to filter by in Special:Watchlist, you may invert the selection by clicking the "Invert selection"-checkbox. While this usually works fine, it does not have any effect when you try to invert the selection "(Main)" (nor does it have any effect if you try to invert the selection "all", but that's not a bug, that's a feature ;)).

Version: 1.14.x
Severity: minor
URL: http://en.wikipedia.org/wiki/Special:Watchlist