how often is the gender blood test wrong?

I just had a gender blood test done last week at 14 weeks. Got the results, it says girl. However, I got an ultrasound that day and she said "it's a boy for sure." Then the baby flipped and she said she wasn't sure. I heard that you can get a false girl that early.. has anyone had that happen? We will be happy to welcome another baby girl but we want our boy already! We feel like its a boy. My husband even went all out and bought so many boy clothes (stupid i know) but there was a garage sale that had all brand new carters boy clothes with the tags still on there for maybe $2 a piece, so he had a good reason!