What happens if you drive a rental you didn't take out?

In general, only authorized drivers as specified on the rental contract are allowed to drive rental cars. This includes the person whose name is on the contract, and any additional persons specified on the form. A spouse is often included automatically (best to ask).

If you drive a rental car that you are not authorized by contract to drive, and something happens to the car, the person whose name is on the contract will be held responsible.