We hired a roofbox (complete with bars and fitting) for the week, packed the car and took care not to overload the roofbox (it contained the travel cot, sleeping bags and camping mats plus a couple of cheapy garden chairs).
It was fine as I drove through London, along the M4/5 and various other routes to Cheltenham but when I was driving at 20mph through one of Cheltenham's less salubrious areas (I'd taken a wrong turning) I gently braked and, as I stopped, the roofbox (complete with bars) scraped down the bonnet and onto a roundabout 
.
Cue one hysterical 5-year-old, a bemused 15-month-old and various puzzled local residents stare at the offending object (and me) as I try to drag it out of the road. Luckily the puzzled local people then realised it was real and came to help me. An elderly gentleman offered us the use of his front garden as a landing patch for the wretched roofbox and I then rang a local friend who came to transport the contents to the Travelodge where we were to stay that night (en route to camping). My DH was at a funeral so wasn't around to help me re-fit the stupid box and I secretly hoped it would be stolen so we wouldn't have to travel with it on our car ever again. Sadly it was still there at midnight when he and another friend went to retrieve it.
My 5-year-old has declared the experience, "the worse day of my whole, entire life, ever" and was concerned that, "Daddy might be put in jail today.... because he might say cross things to the roofbox man". Refusing to travel in an overloaded car (so as to leave suicidal roofbox empty) with two children who wouldn't be able to get out easily in the event that the roofbox pulled any more stunts, I took the girls back to London on the train. We arrived in time to catch a bus across the city that had most of the Notting Hill Carnival revellers crammed onto it.
Moral of the story: hire a bigger car, not a roofbox.