Revocable beneficiary: the owner can change the beneficiary at any time without the beneficiary's consent.
Irrevocable beneficiary: has a vested interest. The owner needs the beneficiary's written consent to change the beneficiary, take a policy loan, surrender the policy or assign it.
Common disaster: the insured and the primary beneficiary die in the same accident, and no one can tell who died first.
California law (Probate Code §224): if it cannot be shown by clear and convincing evidence that the beneficiary survived the insured, the proceeds are handled as if the insured survived the beneficiary. The money then goes to the contingent beneficiary or, if there is none, to the insured's estate (with a community property exception).
A common disaster clause in the policy goes further: the primary beneficiary must outlive the insured by a stated period (for example 30, 60 or 90 days) to collect; otherwise the proceeds go to the contingent beneficiary.