For a screening procedure recommended as a mass conducted preventative measure in otherwise healthy people, harms must be regulated to a better standard than "doesn't happen often". The study that I read of was about serious issue occuring something like one in 120 procedures. It was done at Kaiser. Next time you're enjoying a sausage, take a moment to look at the sausage skins. If I understand correctly, our intestinal walls are quite thin, and even the colon vulnerable.