I had to do the amended W2 thing years ago, when an employer who was six months behind on paying me claimed as of April 15 that they'd been up to date. Since I was cash basis, not accrual basis, the fact they owed me money made not a whit of difference to my taxes unless they'd paid. I never did get the amended form out of them, but I filed the (forget the number) Taxpayer Explanation of Discrepancy, and the IRS was completely fine with it.
That can be very useful