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Employer reported ESPP ordinary income on wrong year's W-2
You mentioned that the 1099B that reports this sale is for 2014, which means that you got the proceeds in 2014. What I suspect happened was that the employer reported this on the next available paycheck, thus reporting it in the 2015 period. If this ends up being a significant difference for you, I'd argue the employer needs to correct both W2s, since you've actually received the money in 2014. However, if the difference for you is not substantial I'd leave it as is and remember that the employer will not know of your ESPP sales until at least several days later when the report from the broker arrives. If you sell on 12/31, you make it very difficult for the employer to account correctly since the report from the broker arrives in the next year.