There's actually a couple of things:
Depending on the plan, you have to put in the "planholder" not necessarily the "Insured" birthdate for processing. This used to be a bigger thing until most states' insurance commissions made it illegal to force that behavior.
Technically though for most plans, you're "supposed" to refuse a fill in those circumstances as they aren't proven to be that person. Obviously, no one does that in practice unless you get a chargeback (Aetna and Blue Cross did that), so it's usually something that corrects at the next plan year.
My favorite ones though are VA birthdates that are different from the patient's real birthdate, because the patient lied on their age to enlist in the military (this was a tacitly allowed practice before 1980 when the EDIPI number was started which cross-references SSN), you might get a person who has a military DOB of an older person but present a drivers license that is younger. We're still in the midst of correcting those since the law changed in 2010 or so, but I can't tell the difference in most people between 75 and 85 in most cases.
