It happened to all of us at least once or twice: we accidently mark all messages as read, rather than just one. Unfortunately there isn't an 'undo' command. The modified date field is not affected either, so it’s mostly impossible to know which messages were marked read. You can sort by the icon column and mark all opened (but not forwarded or replied) messages as unread. You'll end up with a lot of messages you're done with marked unread but it's as easy as pressing Ctrl+Q to mark them read.

If you accidently mark messages read very often, try using Ctrl+Q instead of the mouse to mark messages read. Or get in the habit of marking them completed instead. This changes the modified date and sets the completed flag, making it easy to identify the items you’re done working with.