Couple is married and Mr. makes Mrs. the primary beneficiary of his retirement account.
Mr. & Mrs. get divorced.
Mr. does not change his beneficiary and then dies.
The U.S. Supreme court recently ruled in Kennedy Estate vs. Plan Administrator for DuPont Savings that the ex-spouse gets 100% of the retirement plan because the beneficiary designation trumps the divorce decree .
Moral of story: Always update your beneficiary designations for retirement plans, especially after a divorce.