I mis-sexed a male Ball Python this year that I sold for just under 10k. The buyer (who has probably produced more Ball Pythons than everyone in this thread combined) also mis-sexed it when I handed it to him in person. A few months later I get the call that he popped and then probed it and that it was a female. He did this multiple times to ensure that no mistakes were made again! I had to dig up a bunch of cash and buy it back since he was not looking for females at all. Bummer!
My point is, that it can happen to anyone.