I think it's fairly rare, from a percentage standpoint. Unless they just can't see it because of the way the baby is positioned, ultrasounds these days are very accurate.
Getting another ultrasound just for gender check won't be possible unless there is a problem with the baby (so I hope you don't have to have one!). You can go to an elective place and pay about $75 (at least that's the price in the nearest city around here) to get one just to make sure.
I was worried at my 19 week ultrasound that it wasn't accurate that I was having a girl - I just couldn't believe it, I thought my DH made only boys. I mentioned coming back in to check in 10 weeks (mostly joking) and she guaranteed it was a girl. I still didn't believe it fully until I pulled out the ultrasounds of my boys from the same view - she is clearly a girl! No question.
Do you have ultrasounds of your little one in that area? If so, I would search out both gender ultrasounds on Yahoo or Google and compare them to give yourself peace of mind.