1.You'd probably experience engine knock and misfiring if they were not down into the port deep enough.
2. They could possibly be shot out of the port from the force damaging the hood or other components in the engine bay.
3. Nothing at all. Maybe something tells you that you should just check the plugs and you discover they were loose and you tighten them.
4. The plug flies out and the empty port scorches everything from the heat and flames coming out.
5. Your car morphs into a marshmallow and you are forced to eat your way out
Most likely #1 and #3, although #5 would be awesome except for the fact that you would no longer have a car.