From Redditor u/[deleted]:
Six years ago, I swapped my then-girlfriend now-wife's cat with a more well-behaved lookalike.
She had an all black cat that was extremely aggressive. It scratched everyone, hissed at everyone, and didn't use its litterbox half the time. My wife insisted she could get it to behave better. One week she went out of town to visit her family and I was supposed to go to her apartment and feed it.
The first night I went over, it scratched the sh*t out of my arm. I joked to the cat that it's not special and I'll replace it if it scratches again. The joke stuck with me until I had thought about it enough that it wasn't a joke.
The next morning I went to the local animal shelter. Found an identical cat who was already litterbox trained and acclimated to people, but was a little skiddish (it's old owner passed of a heart problem and the animal shelter people said they think that's why it was skiddish). But overall, it was a lot friendlier and better behaved, and the skiddishness would help it resemble the original cat.
So I adopted it, took it to my wife's apartment, settled it in, then drove her original cat to an animal shelter a town over (I was paranoid my wife would find out if I took it to a local one).
It's been 6 years since then. We got married 4 years ago. We still have the swapped cat. It answers to the original cat's name. My wife knows nothing. She loves this cat and brags about how much better behaved it is. Everytime I see it, I feel like a total piece of sh*t.
TL;DR: My then-girlfriend used to have an aggressive cat that would always hiss and scratch everyone. Secretly, I brought the cat to a shelter a town over and adopted an identical looking cat. We have since gotten married, and she still knows nothing.