Sometimes late Dec Paychecks go out in Jan...and Jan pay ends up on a 2019 W-2.
Thus, you probably(?) didn't have that final pay listed on your 2018 tax return W-2...so you have to report it now.
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or...perhaps you received some kind of bonus pay, based upon your prior work in the company, that was issued in 2019 (that happened to me the year after I left my last company.....I wasn't going to argue about getting more income)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
