If you smoke cigarettes but applied for life insurance as a non-smoker is a very common one.
If you did not disclose your health history properly when you applied for your life insurance policy your claim can be declined.
The good news is that if your life insurance policy is over 24-months old, it is non-contestable. So the claims have to be paid. There is a contestability clause that lasts for the first 2 years you are insured. After that period of time, the life insurance policy is incontestable.
Also many people try to file claims on term life insurance policies that have already ended. Those claims will never be paid. If your life insurance coverage has expired, or you failed to pay your premiums and there is no longer coverage, your claim will be denied.