What would cause this??????

The speedo drive is on the trannie output shaft, connected to the drive shaft. If the car is not moving the speedo drive gear can't be moving or turning the speedo cable. So....something has to be between the gear (trannie) and the actual speedo that is turning the cable....or the speedo just broke in a strange way.

Was the speedo behaving 100% Ok before this? Not bouncing or anything like that? Did it ever show a speed indication while in park or not moving?