Each employer caps separately
Social Security tax applies to wages up to an annual taxable maximum and not at all above it. Every employer applies that ceiling to the wages it pays you, because no employer can see what another paid and none is required to ask.
With one employer that works exactly as intended. With two, the same band of earnings can be taxed twice: each employer withholds correctly against the maximum on its own, and between them they take more than the year’s limit.
Nothing on either W-2 looks wrong
This is why it goes unnoticed. Both forms are right. Neither employer made an error, neither will correct anything, and no notice arrives. The over-withholding exists only when the two are added together, which happens for the first time on your return.
It is most common after changing jobs partway through a year, and it happens to anyone holding two jobs at once whose combined pay passes the maximum. The higher the pay, the larger the amount, and the maximum credit is the whole year’s employee-side tax.
Claiming it
The excess is a refundable credit claimed on Schedule 3 of Form 1040. It is not a deduction and it is not something an employer refunds; it comes back through the return. Most filing software works it out from the W-2 entries without being asked, which is the main reason the amount is worth knowing: so you can check it appeared.
Our wage base calculator takes the wages from each employer separately and shows what each withheld, what the correct maximum was, and the difference.
Only your half
Each employer also paid a matching share on the same wages, and that half is not refundable to you. The credit is the employee side only. This surprises people who have heard that the tax is split, and it is worth knowing before expecting twice the figure.