Typically, severance agreements releasing employment claims are enforceable. People are charged with knowledge of what they sign, unless it is written in a language they are not familiar with or they have a very limited education. Assuming you can prove that the severance agreement was presented as take it or leave it with only an hour to decide, so you did not have the opportunity to read, understand and review the document with counsel of your own choice, you raise some thorny ethical and legal issues. I am not confident they are enough to invalidate the release.
Please note there special rules regarding the validity of releases of age discrimination claims, which require a minimum number of days for you to consider the release and give you the option to revoke the release for an additional number of days. Finally, workers compensation claims cannot normally be released without approval of the worker's compensation appeal board.
Good comprehensive article re federal requirements; note the disability claims references: http://mayalaw.com/2011/03/what-you-need-to-know-about-severance-packages/
A consultation with a California attorney who practices age and disability discrimination would be a sound option now.
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It will be your burden to prove it was under duress. Be prepared to have witnesses and/or correspondence for evidence and you will likely lose the he said she said argument.
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Your question presents a number of fairly complicated issues. I recommend that you contact an attorney to discuss your situation. There are many fine attorneys on AVVO and the California Employment Lawyer's Association. Best of luck
"This contract agreement would not be valid right." I don't see from your post what makes it automatically invalid; as suggested above the burden or proof may well be on you to acquire a judicial declaration that all or part of the release agreement is unenforceable. As others have said, it's multi-part question with complexities, and your other conclusion that the law says 90 days must be given for a lay-off is also worth a legal opinion as well, the rules for WARN type severance benefits are fairly complex. I think you should get your answers as part of an attorney-client relationship.
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