[valdiorn]

I've never not had a contract that stated I had to get consent from my employer if I wanted to take any other job. And I get the feeling that if you got caught double-dealing like this, a company would take it seriously enough to actually sue you for a serious breach of contract. Especially if the other company is in the same industry - this creates a huge conflict of interest and the potential for data leaks, etc. IP ownership and legal issues make this a very dangerous game.

Maybe you get away with this if you're working for small companies that can't afford the legal procedures, but if you're working for a small company there's also much greater risk you'll be found out because you can't disappear into the crowd.