Unfortunately, I'm the reason why people say that. My last baby, I was told at two different u/s (the first with a definite yes, and the 2nd a most likely) that I was having a girl. But, I can assure you that my little one is ALL boy!
What's funny is when I was in that "girl" time, I was the one that was questioning. Though my tech seemed confident and everyone who saw my gender shot pics were reassuring that it was DEFINITELY A GIRL, I was so afraid that I was going to be "the one" that they got wrong. :( Whenever people would ask me what I was having, my reply was always, "The tech says it's a girl. I hope she was right." They'd look at me like I had 3 heads, and say, "Didn't you have an ultrasound?" They couldn't understand MY hesitation. Kinda surprising since your experience is so different.
It was a hard time, but the thing I know from this end is that I survived it. I love my baby regardless of his "parts", and he's just a special piece of my heart now. If it does happen to you, though I seriously doubt it will, you'll love your baby through it. Your baby will bring you through it. And, though you will hurt over the loss of the daughter you "thought" you had, you'll realize babies are babies and none could be better than your own baby!
GOOD LUCK! Try to enjoy your pregnancy despite any fear you may have. It's a special time that is over so fast!