It depends on your state.
Probably the worst thing that will happen is that somebody is going to have to pay the back registration penalties. Might be good to find out what those are before you sell the car and have a very unhappy buyer find out he owes hundreds of dollars on registration. You can call DMV and ask for this info. They won't come to your house or anything. One way or the other they'll get their money.
If you want a "cleaner" sale, at least put the title in your name, even if you don't register it. Otherwise the buyer might question who owns the vehicle---which is true---if your name isn't on the title you have now, whoever has that title in their hand is the legal owner. So if you lose the title you are kind of screwed on this one.