There is no "I replied to this message" flag as of yet. Is this a request for one? If so, would you like it to be just a flag, or would you like to know the actual date/time that you replied? If the latter, if you reply more than once is just the information for the most recent reply sufficient?
Thanks for finding this bug. It's actually a message for me, telling me that *I* should check *my* MySQL manual.
But I can tell what happened. In the row that begins "Mark Selected" you clicked on "Read", causing it to try to mark all selected messages to the "read" state, meaning that you'd already read them. Since you had nothing selected there was nothing to place between the parentheses of the MySQL command, and empty parentheses are illegal syntax.
What *should* have happened is that it should first have checked if you had anything selected and then done nothing. I'll fix it tonight.