Flying boats! I have seen it happen and it has happened to my kayak on one occasion. The sinking feeling you get as you see your boat fly off the top of the car isn’t very nice.
In the case of my kayak there was a flapping sound, followed by the clunk of the buckle hitting the car roof followed by a whoosh as two kayaks flew off down the road! Fortunately on both occasions no one was hurt and the damage to the boats minor. If it had been later in the day it could have been a different story.
Tying your boat onto the roof rack correctly is critical. It could save both your kayak and car from serious damage and keep other unsuspecting motorists that little bit safer. Using good quality straps or rope is a must and if the strap/rope is worn then they must be retired.