Related to MBS-4904 and reproducible for me in Opera 12 and Chromium in Linux (Firefox has different issues). Haven't tried any others yet, but IIRC also Opera 12 in OSX:

• Load http://musicbrainz.org/search/edits
• Focus "Please choose a condition"
• Press the down arrow to scroll through the list

Expected result: You can use the arrow keys to move up and down the list.
Actual result: As soon as the first option is selected, focus is shifted away from the dropdown to the text field created for the first option, so you have to tab back to the dropdown to continue trying to move down the list.

The same thing happens when trying to change options or change things like "is" to "is more than":

• From the text field focused previously, tab backwards to the dropdown it added
• Press down again

Expected result: As above
Actual result: As above