You find out your contract has an invalid clause. What now? Contract null and void and you pay back all the wages received? Or just stop being employed from that moment forward?
Apologies, I never thought you were disingenuous, it's just a busy time of year :)
In that circumstance, if both parties are still happy with the terms they would presumably sign a new contract minus the unenforceable terms.
If the company opts not to create a new contract, I would say it counts as firing the person without cause.
It is an interesting question though, especially as it pertains to future incentives. Something like the person having earned a bonus that hasn't been paid out yet, the contract is found invalid so the bonus part is scratched out.
Perhaps the individual deserves a choice in that case, whether to proceed without a contract or under the terms minus the unenforceable parts, or maybe something more punitive to the company like the individual keeping any rights they got and the company losing theirs.
I still think it's a workable idea, but you do raise an interesting point that's probably worth adjusting my idea around.