I've returned a pet.
Three years ago we got a cat from the Humane Society. He appeared to be a sweet cat, age 3, male. We were a bit concerned because we had another cat at home, so they brought another male cat into the room to be sure that "jake" the cat we were planning to adopt would seem okay with another cat. For about 15 minutes he was in the room with the other cat, and everything seemed fine. We took Jake home, and there was growling and hissing between him and our other cat, which did't surprise us, normal for two cats meeting.
Fast forward three days: Dh walks into our bedroom where Jake was sitting on our bed. For no apparent reason, Jake attacked my husband, biting his arm, breaking the skin and causing a lot of bleeding. DH had to go to the hospital and get antibiotics.
After that, he started terrorizing our other cat, who cowered anytime Jake was in the room. There were lots of fights between the cats.
Then Jake scratched both my children viciously, multiple time, unprovoked. Then I was in at the dining room table and he came over to me and swatted at me so hard he knocked my glasses right off of me.
For three months we tried everything we could to get him to relax and calm down. Nothing worked. Three moths after we got him I returned him to the Humane Society. It was no fun having a pet that everyone in the house was terrified of. I felt terribly guilty (and still do), but there really was no choice.