On Super User, I noticed that the dialog box for flagging questions appears correctly only when I refresh the review page. The second time I flag a question without refreshing the page, what I see (after selecting "it doesn't belong here") is the following.
If I click on "Flag Question," I get an error message.
I have tried with Firefox 12.0 (I know, it's not supported), and Google Chrome 18.0.1025.142; I had the same result as with Safari.
On Google Chrome I noticed that if I first flag for moderation attention (selecting other, and entering a custom reason), and then I flag selecting "it doesn't belong here," the dialog box appear fine. It is only when I flag using "it doesn't belong here" twice in row that I have the problem I have described here.
The revision reported on Super User, when I tried with Firefox and Google Chrome, is 2012.4.3.2076.