Accuracy of the fetal in a sonogram?

at my 20 week scan the doctor gave me a paper that said its a boy (i didnt want her to tell me cause my man had to work and couldnt be there and we wanted to find out together.) and a week later i got dehydrated and needed an emergency sonogram because they thought the aminotic fluid was low, (but it wasnt it was fine!) and that other ultra sound lady said "its a boy" BUT!!! i didnt c anything... well she showed me but it looked like a foot to me! i have mad boy clothes and would be very disapointed if its a girl, has anyone been told its a boy twice and it was really a girl? or vice versa? the reason i ask is because i have re-occuring dreams that i give birth and its a girl,