I have woken up during a colonoscopy, so it does happen. They don't really need to put you under as deep as some other kinds of surgery. A few days ago, I had a septoplasty and it was a much deeper anisthesia than I have ever had for a colonoscopy, so I know there are some differences. It took about 30mins or so for me to be able to talk without slurring and start to sit up properly, where as I am almost immediately awake after a colonoscopy.

When I was on the table right before my last colonoscopy, I asked the anesthesiologist why I might have woken up last time. While she didn't go into detail, she mentioned that it might have been for the last doctor to check my responsiveness and it likely wasn't a mistake.