When hauling things on the roof of your vehicle make sure you check everything securing the cargo down a million and a half times. Also, be paranoid, if you hear or suspect the slightest thing is not right, stop on the side of the road to rigure out whats wrong.
My girlfriend and I just figured all this out the hard way. We were returning from a day on the lake with two kayaks when we started slowing for a stop light and both boats when flying off the front of the car. We had checked everything over and didn’t notice anything odd till it happened, we didn’t have any such problems on the way to the lake. No one was in front of us, and the boats were relatively ok but one of them did put a small dent in the hood of my girlfriend’s mom’s jeep. Not fun.
Again, make sure you check absolutely everything over, and over again. And most importantly, never do it with someone elses vehicle.