What CAN go Wrong
Oversheeting the spinnaker makes the boat less stable and heel over more. In windy weather this can cause the boat to screw up into the wind in a broach and it is invariably accompanied by a lot of sail flapping and an alarming amount of heel.
The main and spinnaker sheets need to be eased to allow the helm to steer downwind and get the boat back on its feet. Releasing the kicking strap will lift the boom and allow wind to spill out of the head of the main. This is important if the boat has heeled so far over in the wind that the boom is dragging in the water.
A common mistake is to have the guy too far forward with the sail billowing like washing on the line. The boat will heel over more than with the correct trim and you’ll need to use more helm, which is slower.