Are you sure you're looking at a W-2 form rather than another type of income form, like a 1099-R? These are standardized forms, and box 12 of the W-2 is not state tax withheld (that's box 17). However, on the 1099-R, box 12 is state tax withheld.
Often folks refer to all their income forms as "W-2s" even though they have different form numbers. Take another look at yours, just to be sure.
