For details, see:

http://forums.musicbrainz.org/viewtopic.php?pid=13412

Steps to reproduce:

You are on the "tracklist" page
Hit "add disk"
Enter a disc information in disc 2
Delete disc 1
You should have a single disc, it will be labeled as disc 1
Hit "next>>" to "recordings" page
Hit "<<previous" to "tracklist"
You now have an empty disc 0, and your disk is labeled disc 1