Quoting wannaknow:" Anyone been told you were having a boy and then had the baby and it was really a girl? Just wondering ... [snip!] ... paranoid that it could be a girl, and I will have all boy stuff in my nursery. Has this happened to you or someone you know?"
This time every u/s has been girl, with my son my early u/s they said girl but he turned out to be a boy. I wasn't sold so I had already bought gender neutral things. Most of the time though if it was a mistake it was mistaking a boy for a girl, usually if they say boy it's because they saw a penis or testes. So I think you should be good!