Anesthesia in particular. I've had some family members who were never quite the same after general anesthesia. Not as mentally sharp, more forgetful, etc.

But they should be considered proportionally to their probability times severity, and in this case, the probability is small enough to be a rounding error, so you really should worry about other things first, like getting killed when driving to the dentist's.

According to [1], medical errors are actually the third leading cause of death in the United States, so it's hardly a small probability.

1. http://www.forbes.com/sites/leahbinder/2013/09/23/stunning-n...

You should count only those medical errors which are relevant to your visit. A dentist probably won't accidentally damage your heart with a scalpel.