I was playing VF one day and needed the candy trophy, so I bought a piece and gave it to one of my virtual kids who happened to be reading email. I got a notice saying "unknown string ID!!!" What's up with that?

That minor bug has been reported many times, although it's completely harmless and avoidable. It only occurs when you praise a peep, by any of the various means, for reading email. Since it doesn't cause any problems with the game, it can be safely ignored.

Ok, thanks! Now I know what's going on. I also noticed that when you give them candy or praise them while reading email, it doesn't affect their mood (eg, doesn't raise it or anything). Better to wait till they're done reading and then praise them.

Reading email can only be initiated by the player, not the peep. Since the main purpose of praising them is to encourage peeps to carry out that particular action again, the game is confused and throws up the error message.