This is one of those "What types of forms are showing withholding? " questions.
Usually, any withholding is a number you would have had to enter yourself in the wrong field when you went thru the interview.
1) Certainly, there could have been a few foul-ups for downloaded forms. For example, if you downloaded a tax form, and the "provider" messed up the tax file layout, then the wrong numbers could get in certain fields...but, you were supposed to confirm all the numbers...on ANY import yourself because TTX can't know a provider improperly formatted certain numbers.
2) For a several years running, Los Angeles public service 1099-R forms were mis-numbered in the later box numbers....and users had to make sure they put them in the "proper" boxes in the 1099-R form in TurboTax. That was Govt screwup in not using the proper IRA approved 1099-R forms. (Don't know if, or when they ever fixed that).
3) Maybe you used the "picture of W-2" procedure to enter your W-2 form that year. It was then up to You to check that all the boxes were entered properly (I would never trust such a procedure)
4) Sometimes people who exercised employer's stock options will double-enter their withholding, putting the withholding in with their 1099-B/8949 entries for the exercise and sale......when it's already shown on their year-end W-2. TTX can't know that you did that.
5) And, while not an actual "income tax" withholding issue, some people overstate their SS withholding by not noticing that they assigned BOTH their own W-2 and their spouse's W-2 as belonging to only one of them...resulting in an improper SS refund for any total amount in box 4 that is over 7979.40 (for 2018). That is a user-entered error that TTX can't know you've done. TTX always presents a page after the entry of a W-2, showing the W-2 and who it belongs to...one for Mary and one for John....if both showed for Mary or both for John, you should have corrected that.
6) there's also an entry section on the Deductions & Credits page where you can enter any "Withholding not already entered on a W-2 or 1099 " You might have double entered some withholding there.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
