Buddy of mine worked (as an engineer) for a well known firearms manufacturer. Upon his resignation to go work for another well known firearms company, the boss called him in the office and told him that in order to get his last 2 weeks of pay he needed to sign this document. He said he would look it over and consider it. He looked it over and the synopsis was, "every bit of firearms knowledge you have is because of us, any design you create in the future is because of that knowledge. Our company owns all rights to those designs no matter who you work for." He talked it over with a lawyer he knew, the lawyer said, "DON'T sign that".
The boss was pretty upset and threatened to withhold pay... he still got paid (I think the boss realized that withholding pay would only open him up to a lawsuit).