In this day and age it would be unlikely they're wrong because they can be sued if they are. So if theyre not 100% sure they'll say that (I think they say that regardless) or they won't tell you. I had three u/s for DD2 and all of them said this "it's a girl I think, but I can never be 100% sure). This was to cover them, but of course it was a girl, otherwise it would have been a boy missing his important parts!!
Also if you have an early scan, it's best not to find gender (ie at 12 weeks). My sister had this, they said boy, but then at 20weeks they said girl xoxo