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When you select 'other' without entering anything into the text box and then click 'not an answer', the Flag Answer button is not enabled.
Clicking 'not an answer' again properly enables the button.
(I am using Firefox 5.0 on Windows 7)
This was fixed back on August 22nd, thanks to Jarrod Dixon. ♪
Flagging should work normally from now on, no more button disabling.