i think you stand very dodgily! In fact, im sure you must be breaking some sort of law as you know you are not entitled to this money, yet you are still depriving the company of it. I would think when they ask for it back you would find it very difficult to say you didnt realise and would therefore be in no position to negotiate a payment plan back to them, and will have to give it all back at once.
As is said in all these question: imagine the situation were reversed and you had accidentally put some money in someone elses bank account - would you expect them to give it back when you realised your mistake?