I haven't seen it happen that often but there have been one or two instances that I've heard of.
This is just based on my perception - it seems more common for a girl to be mistaken for a boy. All babies' private parts are on the outside of their bodies until a few weeks before the a/s scan. At that point, girls' parts tuck inside their bodies. That's why you don't want to make a guess on the sex too early. Sometimes, at the A/S scans, I think the technicians will see a little bit of girl parts and mistake them for boy parts.
However, I'm pg with my second boy and it has been VERY obvious in both scans that we were having boys. No question about it at all. It's hard for me to imagine someone mistaking a boy for a girl by 18, 19, 20 weeks, unless they get a very poor shot of the private parts and wager a guess on that.