One drawback of the buses is they typically do not stop at the Mexican immigration building when crossing the border back to the U.S.. No Mexican official will ever be collecting them on the road out, it's up to you to stop and turn them in.
I've heard of a few members on other forums who say they were fined but they also say the officials did not know about the expired permits until they fell out of their passports where they had left them.
My advice would be to throw them away or hide them and say nothing. If on your next entry they say something about not turning in a past permit (which is doubtful), insist that you did turn it in. When I've turned mine in they have never entered anything on the computer or filled out any form or stamped anything (at least not in front of me) and I've never been given any kind of receipt. For the car permit yes, for the tourist permit no. I always get the feeling the agent waits until I walk away and just tosses it into the trash can.