Can Ultrasounds be wrong?

Ok, so I know I just put a post on that we were having a boy but that is truely what we thought up until today. So here's the scoop- Aug 15th our last Dr apt we had an ultrasound because I had been bleeding so much. The Dr told us it was a boy, (oh and that everything was ok) the pict we saw looked like a boy and were we suprised at all? No, cause we don't get girls in my family. My folks have 11 grandsons and zip for little girls and Jer's side just has boys too, even the cousins just have boys. So girls just don't happen. Well today we went in again, another ultrasound (cause I'm 19 wks along) and the gal doing the ultrasound asks if we want to know the sex of the baby so we explained that we already know it is a boy because of past ultrasounds and she kindly informed us they(the ultrasounds) were wrong and it was clearly a girl. Yes, we have proof, but do I truely believe? Not fully. Not until I have a baby in my arms and can check for myself. So for now it is a girl. Next month, who knows maybe I'll go for twins!