The most common reason is that you worked multiple jobs, or changed jobs.
Each employer calculates Social Security independently. If you earned more than $132,900 combined from BOX 3 if all your W-2 you will have overpaid.
This is not a problem. It will be credited to you on your tax return on line 18 (d) of your Individual Tax Return.
I think TurboTax is incorrectly calculating Tier 1 tax.
We paid the full 8239.79 as required
TurboTax calculated that we should have paid only $0.00 - clearly incorrect
TurboTax says our excess withheld is 8239.79
Please Fix TurboTax!
RAILROADERS:
Just spent an hour and a half on tax support:
If you have excess Tier 1 tax paid because of more than one railroad job, we had to do the following to resolve TurboTax problems with calculating excess Tier 1 Tax and applying the credit to the 1040:
For both W-2s, Make sure you enter RRT1, RRT2 and MED in the same order in box 14 on TurboTax (each box 14 line should be the same on each W-2 (Line one for RRT1, Line 2 RRT2 and Line 3 MED in TurboTax).
After you get all your box 14 items from each W-2 into the proper order on TurboTax, ADD A LINE UNDER THOSE and write "RAILROAD COMPENSATION". In the $ amount box put your box 1 wages from your W-2 and then select Railroad compensation in the pull down menu. TurboTax will then correctly add up how much Tier 1 tax is due, then tell you what you properly overpaid, and then put a schedule 3 in your tax forms and give you the proper $ credit to this year's taxes. You would still have to file a form 843 for excess Tier 2 tax if you paid that.
We never had to do this in TurboTax before. I don't know if it is a glitch or a feature, but it is not well done on TurboTax's part.
Hope this helps. It worked for us.