The 2 sides of a cassette had been added as 2 mediums. I manually appended medium 2 to medium 1 and subsequently tried to remove medium 2.

1. http://musicbrainz.org/release/f1413e4f-8638-42f9-9b72-21cae4eb1426
2. Edit -> Tracklist -> Advanced -> Press x to remove medium 2 -> Enter edit note
3. "Internal Server Error" [...] Error message: (No details about this error are available)"

I did the same once before on http://musicbrainz.org/release/67e1bd58-314e-4947-8474-514285fc1243 For some reason, it did work in that case. However, I was able to approve my own edit: http://musicbrainz.org/release/67e1bd58-314e-4947-8474-514285fc1243/edits