1. Edit any recording with instrument ARs on it (I tried http://musicbrainz.org/recording/bfaad082-59ad-4522-8339-9d3a0cde0ee7/edit)
2. Click on the icon to edit one of them.
3. Change the instrument or add a new one.
4. Press Cancel.
5. The relationship is still changed.
Other relationships do not seem to have this issue.
Side note: Now that these recording/work/... editors allow a bunch of things to be added at once via the mini-RE, perhaps they warrant a Cancel button and are-you-sure-you-want-to-leave when navigating away?