If a toolbar section with a dropdown box (currently only the "headings" dropdown in the "advanced" section) is opened, closed, then reopened, the dropdown will appear underneath the editing textarea, making it unusable.
I've observed this personally on FF 3.6.3, and one other user has noted it on FF 2.0.
Version: unspecified
Severity: trivial
URL: http://en.wikipedia.org/wiki/Wikipedia:Requests_for_comment/May_2010_skin_change/Bug_reports#Toolbar_dropdown_boxes_occasionally_open_under_the_edit_window