I once did an internship with a BigTechCompany, and after my internship was over and I went back to university, they kept paying me! Every two weeks, I got another paycheck direct deposited. As a starving student, I have to admit it was not easy to just ignore it sitting there in my bank account but I knew if I spent it I'd eventually be in a world of hurt. I tried to contact BigTechCompany, but evidently their HR department was as incompetent as their payroll department--they didn't understand what I was talking about and kept saying since I don't work for them they would not discuss internal payroll issues with them. So month after month, my bank account grew and grew, and I'm sitting in class eating ramen noodles.
Finally, after about 8 months of this, I got an extremely threatening letter from BigTechCompany's lawyers informing me that due to a payroll error, I was overpaid, and they demand I immediately return the money that was not mine or face legal consequences, blah, blah, blah. Of course I sent it back, but what a wild and crazy way for a company to treat people when it's entirely the company's fault.