In Safari Version 5.1.7 (7534.57.2) and Firefox 12.0 on OS X, I can reproducibly trigger the following error message:
- Click a link that opens a styled popup (not a dialog box), like close or flag.
- Close the dialog by pressing Escape or clicking the X button in the upper left.
- Click a link of the same kind as step 1's again (i.e. any flag link on the same page suffices if you selected flag before).
Workaround: Click it again, i.e. a third time.
Still, seems to be a bug.