I must admit that I've been fooled by a cat too!
In 1993 I got a tiny black and white female(or so I thought). We named her Chelsea. The kitten was super fluffy, so it was hard to get a good look "back there".
She went to the vet after we'd had her for about 2 weeks. The vet kindly pointed out that Chelsea was in fact a male.
At this point Chelsea was responding to his name, and so we decided to keep the name. In his 6 years he never seemed to mind having a female name!