http://beta.musicbrainz.org/edit/18564484 Here I edited the disambiguation comment and pressed enter. I didn't even get as far down as the IPI field. When I submitted it, it changed the IPI.
Editing the artist again shows "MusicBrainz::Server::Entity::ArtistIPI=HASH(0x11c9a410)" in the IPI field (see
MBS-5050). I'm guessing somehow turned that into an IPI instead of giving an error.