Steps to reproduce:

1. go to http://test.musicbrainz.org/release/f77eee87-5d73-3a08-8637-8457cfff6053/edit (or any release edit page, presumably)
2. hover any track number, notice icon for draggable
3. click it
4. track is removed from list and placed into an overlay
5. keep clicking other tracks
6. end result is a mix of tracks in an overlay doing nothing

I assume this is a half-finished feature for track reordering or something. Attaching what it looks like after clicking a bit in Opera.