Nub theory/angle of the dangle

At our 12 week (first tri) u/s we were told by our tech that she was 85% sure we are having a girl, she also mentioned she had been doing this for 8 years. I'm pretty excited about having a girl but I have heard stories of women having the u/s re-done at 20 weeks revealing the opposite sex. Has this happened to anyone recently, I emphasize recently because technology is always advancing.