The meaning of the "flag as inappropriate" icon (tiny plus sign and flag) is unclear; there's no text unless you hover long enough for the title tooltip to appear. And if you click it unintentionally (to find out what it is, or because you misunderstand the meaning) there is no way to undo.
This is definitely true. Luckily, in the moderation panel I can revert the accidental flagging. Going to call this a won't fix, but agree that this is not optimal.