Sorry there's no log, but this should be very easy to reproduce.

This greater undead is really cool, but I go out of my way to avoid using this because if I get it, I'm married to it - til death do us part. Is there an easy way to make this banishable? Maybe quitting or casting greater undead somewhere else dispels it?

I wasn't sure this was a bug until I searched the forums and found this: http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=4&topic_id=121932&mesg_id=121959&page=

Also, every time I cast greater undead in a poltergeist graveyard, I always get a "two greater undead are more than you can control!" message after the poltergeist appears.

NCR, you can use the BANISH command to remove a poltergeist from your group. Thus: "> banish poltergeist".

It's a strange case, so let us know if there are any further issues.