Having owned hundreds of snakes, I have only experienced one gender mix-up to my knowledge, and it was on the very first ball python that we bought for our son almost nine years ago from a pet store. We bought the snake as a "male" and finally discovered it was actually a female three years later when I started becoming suspicious of the large body size and small spurs. Sure enough, she probed female. So, "Jake the Snake" became "Belinda" and went on to become an excellent breeder for me, laying clutches of 9-10 egg clutches. We still have her today. My kid was initially shocked and disappointed by the gender mix-up, but he got over it pretty fast.