Well, you'd probably get a parking ticket.
Initially you'd only want to use drop-off self-parking where you know parking is free: in office parks for example. Or where your car has a parking decal or your plate number is registered, etc.
However many urban areas already have app-enabled parking meters. That lets you pay from your phone. Given sufficient demand, it's easy to imagine integration with the car itself. Say your car sends its plate number to the parking system, along with its location. Maybe it can read the stall number from a camera, or maybe using something like RFID or bluetooth. The parking system would look up your plate number, find your account, and send a notification to your phone. Then you'd approve payment, and you wouldn't get that parking ticket.
Most of the pieces for a system like that are already available — but the problem doesn't exist yet, so neither does the solution. Maybe that's a business opportunity for some enterprising person.