Yeah, a big one, the contacts won't make or break at exactly the same time so during switching you'll violate the relays ratings, if there's an induction involved the relay will die a very early death.
Why not just get a 20amp relay? They're not so expensive as to justify using a 10amp relay in this manner.
EDIT: 13amps is only slightly over the 10amp rating of the relay it might work okay in that manner. It MIGHT be okay, but I honestly wouldn't trust it if it's connected to something important.