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I disagree in concept with the other answers, but as a practical matter, it is unlikely you could prevail.
Liability for the negligent administration of employee health plan benefits is an issue governed by a federal statute known as ERISA. ERISA makes it virtually impossible to get damages for poor administration of a health care plan. You can usually only get the policy benefit you should have received if you prove that the conduct of the plan administrator was not just wrong, but an abuse of discretion.
Depending on the nature of the "mistake" and the benefits lost, you may or may not have an ERISA claim. However, damages other than the policy benefit are not available to you. If the amount in controversy is large enough, you might be able to get an ERISA attorney to assist you.
You should locate and consult with an ERISA law attorney as soon as possible to explore your facts and determine your options. I would suggest you look either on this site in the Find a Lawyer section, or go to www.cela.org, the home page for the California Employment Lawyers Association, an organization whose members are dedicated to the representation of employees against their employers.
Good luck to you.
This answer should not be construed to create any attorney-client relationship. Such a relationship can be formed only through the mutual execution of an attorney-client agreement. The answer given is based on the extremely limited facts provided and the proper course of action might change significantly with the introduction of other facts. All who read this answer should not rely on the answer to govern their conduct. Please seek the advice of competent counsel after disclosing all facts to that attorney. This answer is intended for California residents only. The answering party is only licensed to practice in the State of California.
This is a reports of the same question asked a couple days ago.....no different result in answers from this batch of answers.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.