I've noticed this several times - hit 'Alert moderator', type a couple of words, click 'Alert moderator' button and it turns grey and the page just freezes. Click Cancel and refresh the page - 'unauthorized' message comes up. I'm guessing this happens when a moderator gets to the post faster than I report it (sometimes I open a post but then work stuff comes up and the post stays open for a while).
Shouldn't there be some kind of error message instead of the web page just going to la-la land? In case it matters I use IE 8 (which is highest available for XP).