This entire riddle is based on the premise that it had already been eaten, and Marcus [i]was[/i] saving it for later prior to its consumption. If you cite that statement as a valid proof of Marcus lying, you may as well say that none of his friends could have eaten the cake based on that statement alone. After all, he couldn't have been saving it for later if one of his friends ate it, right? [spoiler]I do understand that for the riddle to work it must be implied that he meant one of his friends had eaten the cake, however, the wording that Marcus uses in his statement makes every possible solution to the riddle contradictory.[/spoiler]

Ah, I see. I'll have to fix that puzzle later, then. Thank you.