Had my dad's bike on the roof and I had to stop off at the market to pick some stuff up on my way to dropping his bike at the LBS.
Doesn't the f****** market have one of those aluminum bars that restrict the height of the vehicles that may enter and park in their parking lot? I never even thought of it as I've entered that parking lot a hundred times without any problems. Luckily all that bent was the bike rack.
So - lesson learned for you Al.canoe is that if you have to put it on. ALWAYS remember you have the bike on the roof. Put something on your dash to remind you especially if you're going to enter any parking lots/garages, etc.