I have bought some sympathy fish. Although I did think about it for at least a month (in most cases) and made sure I had room for them, and a back up plan if they don't get along. I used to work for Petsmart, so every Thursday (the day they got fish in), I'd always come home with an impulse buy of some cute or sad fish.
I had almost bought two bettas that had almost killed each other in one cup! I came into work and I was straightening out the betta cups (I'm a really neat person and things like that have to be right. I don't work there anymore and I still fix the cups!). I saw this one cup that had two bettas in it! I freaked out! They were torn to shreds and they looked horrible. I had immediately seperated them, and thought that they would probably die. I didn't save them though because I had 5 bettas at home, another two wouldn't be a good idea. The saddest thing is that they are still there. They are alive, and their fins grew back, but their fins look all mangled, so I don't think anyone will ever buy them. I also asked everyone working there that day if they knew anything about it, and of course, no one did. I doubt a customer did it because those bettas are in an area that is watched 24/7. It's just sad.