I adopted Addison from the humane society. The card said he was a gray male. The adoption girl picked him up and pointed to where he was neutered (didn't see anything, but knew nothing of cats and just nodded as she talked). I took him in for vaccines that week or the next. The vet tech said that cats his color are normally female (turns out gray was actually dilute tortie) but looked... said "eh, I guess they were right." Went in for boosters a few weeks later. Different tech, same thing... supposed to be a girl but I guess he's a boy.
A year or so passes. Although the more I read about dilute torties (and all the wonderful people here kept telling me!) I swore he was a boy because so many people checked and called him a boy. I knew him as a boy and loved him as a boy. I started working at my vet's office (different than where I took him after adoption) and had brought him in for something. It started again. "Um, I hate to say it but this is a female" and I would argue... I've been through it a few times and always the same. Should be, but isn't.
After several minutes of patient poking and prodding she was certain - my dear boy was a girl.
I cried after she left the room. I was still in denial.
The next few weeks I tried to let it settle in. I tried to say "she" and "her" but it just didn't feel right.
So for the next few years of his life up until the day he passed away Addison was always my sweet boy. My vets and coworkers joked about it but would still ask how "he" was doing and told me they were sorry when "he" was getting sick and dying.
The vet specialist I took him to got the whole story and laughed about it with me. I would still have him listed on files as a female (medically, that part mattered, but I could still call him what I wanted). So when they sent me a sympathy card after he died... the doctor who sent it filled in a little note that she was sorry for what "she" had gone through. It actually hurt a little - not really sure why.
To this day, when a kitten or cat comes in that the owners think is one sex and turns out to be another, everyone jokes that the "gender confused counseler" can come in and guide them if they want.