None of mine were wrong.
When I had DD1 he wasn't too sure, because she had her legs crossed in front of her, but he couldn't see 'any extra bits hanging out', as he said, so he guessed she was a girl, but couldn't say for sure.

With DD2 we wanted a boy, but he was pretty sure she was a girl. I got him to check twice.

With DS I had the u/s & was told a boy, but she couldn't say 100%. So when I had to have another one coz he was stubborn, The lady who did the 2nd one actually showed me his boy bits. It was the most beautiful pic I'd ever seen!
When I had him I was told there were a few mistakes made with the sex recently.
I've heard that unless they actually see a scrotum they will say its a girl. So its possible it was hiding, then he got a quick view.