Well golly by gosh, doctors are human!!!
Should it have happened? Definitely not, but given the number of surgeries world wide it WILL happen at some point. Yes, that is why you get marked in sharpie and that's also why you have a pathologist check on everything ever remvoed from a patient.
I was shadowing in path one time and the doc was telling me about receiving and document a gallblader one time. The next time she was given another case for the same patient who, you guessed it, had a gall bladder removed. The first thought was, who screwed up. But after going back through the records it was actually confirmed that the patient had 2 gall bladders.