If it is the registration from the USA, nothing I went from Mexico City to Texas and then to California with the toads expired tags in 2016, at one point I had a Texas State Trooper behind me at the Falfurrias check point behind the toad, I thought for Shure I will get a ticket, he just passed me and I drove in san Antonio two months with the toad and then on to California, I misread the year 15 for 16.
On the other hand if it is the TIP (Temporary Importation Permit), maybe loose the deposit, get a fine it is up to the border people to decide and that is anybody's guess, last I know they would confiscate the vehicle or nothing happens and they just take the sticker off and home you go.
Good luck!
navegator
