Just select any artist with an IPI code. Click the Edit tab. The IPI codes field will show something like this instead of the existing IPI code:

MusicBrainz::Server::Entity::ArtistIPI=HASH(0xd126dc0)

Submit the edit to that artist and the resulting IPI code will be mangled. I did this when attempting to add an additional IPI code to an existing artist, but I presume this would occur on any artist edit where there is an existing IPI code:

http://musicbrainz.org/edit/18751190