Your will does not control your 401(k).
It does not control your IRA either. You can write the most careful, expensive, lawyer-reviewed will in your county, specify exactly who gets what, sign it in front of witnesses, and none of it will touch your retirement accounts.
A form does that. A form you filled out at a desk, possibly during onboarding, possibly in 2009, possibly while a benefits coordinator waited.
That form wins. Every time. Against the will, against your stated wishes, against what everyone in the family knows you wanted.
Courts have upheld this in cases where the outcome was plainly not what the deceased intended. An ex-spouse named in 1998 and never removed inherits the account. The will saying otherwise does nothing.
Here is the uncomfortable question this article is built around.
Can you name, right now, the beneficiary on every retirement account you own?
Most people cannot. Some cannot even name all the accounts.
That gap is where the damage happens, and it is entirely fixable in about an hour.