I had a roof rack on my1985 Rabbit GTi, and I was pretty paranoid about low overhangs, garage doors, etc. So, I managed to never scrape a bike off the top of the car.
But, one day I did have a bit of a scare. I was driving to work, and I had my mountain bike on top of the car. As I made the left turn onto Parker Road, from Quincy, there was a loud bang on the roof.
I cranked open the sunroof, and there was my bike, lying on its side on top of the car. I reached up through the open sunroof and held the bike in place as I pulled over on the shoulder of the road.
The quik-release skewer on the rack had broken, and that had allowed the bike to fall over. I pulled the QR from the rack on the other side of the car, replaced the broken one, and clamped the fork into the rack. I kept my eye on the bike, the rest of the way in.
I sure was glad that, for once, I had fastened the little strap which held the rear wheel in the rack tray!