'Clear air' turbulence - the plane drops - you effectively stay where you are - and hit the ceiling with a wallop
Keep your seatbelt on?
CAT is a problem because you can't see what causes it; as there's no obvious geographic or weather event that causes it if there are no pilot reports of turbulence and no clouds formed to suggest turbulence it takes everyone by surprise. It's probably the air risk closest to rogue waves (if that's what they're called) at sea.