I started to regain consciousness once during surgery as the anaethetist was trying to use a "light touch" and got it wrong. That was horrifying, as I was aware of what was happening but couldn't move or do anything to alert the doctors to the problem. Scary. In the end my panic sent my heart rate and blood pressure soaring so they realised and knocked me out properly and were able to complete the surgery successfully. It's left me with a fear of GAs ever since though, which is difficult given the state of my health and how many operations I need. The last surgery I had the top consultant anaethetist in the area supported by two other consultant anaethetists, to make sure I was ok.
I've had other, minor, complications during and after surgery but nothing to complain about really.
My mum used to work as the chief admin of a hospital though and she says the number of complaints about surgical mishaps was astonishing! The worst one she had to sort out was when a surgeon left banana peel inside a patient....