This is with Opera 10.10 when editing a recording, e.g. http://test.musicbrainz.org/recording/e5c4302b-8f2b-46ae-afa5-0c809fcefead/edit but occurs elsewhere when using the artist credit editor too.

In step 1, I clicked on ">>", which gives me a new disabled button just off the left of the screen
In step 2, I clicked on "<<", which disables the button I just clicked in favour of the new button that appeared in the previous step
In step 3, I clicked on ">>" again, which pops up the artist credit editor in the wrong place. Oddly enough, the positioning bug here only seems to happen when there are multiple artists in the artist credit.