Do you have an entry called 'Gettysburg'? Is there anything unusual about that entry?

Weird... I am assuming that the categories to which this entry is assigned all have labels, yes?

Just upgrade to MT 2.0.  When I attempt to rebuild on one of my blogs, I get a

An error occurred:

Build entry 'Gettysburg' failed:

MT::App::CMS=HASH(0x822f178) Use of uninitialized value at /home/virtuals/subproc/subproc/cgi-bin/MT/lib/MT.pm line 309."

I saw a similar error elsewhere that related to a stale cookie, but removing my MT cookie did nothing.

This does not happen on all of my blogs, just the one.

Yes.  I noticed that that entry was the first entry with an alternate category.   For example, if the 5 most recent entries were in the category "one", and Gettyburg was in category "two", the build would fail on hitting the 6th entry (which is gettsburg).  Toggle gettysburg to category one and the problem sort of cascades down to the next entry with an alternate category.

There isn't anything odd in that particular entry.  And this only happens in the one blog.  Others remain are unaffected.

yeah, they all have labels.