Caused by the patch for
MBS-5388. There're a couple issues, which can be reproduced:
1. Go to http://beta.musicbrainz.org/release/61bffce0-2af5-3486-9062-0174628423c3/edit-relationships
2. Click on any recording<->recording relationship.
3. Click "Change direction" once. The relationship's link phrase doesn't turn orange, even though it was changed (a regression).
4. Click "Cancel." The relationship now has a red underline to indicate an error. If you click on the relationship again, it thinks the entities are the same (they aren't, and the error will never go away).