"What if your client comes in to do a 2020 return and says, "By the way, here's another W-2 I forgot to give you last year?"  

And then there are the joint returns where this year's spouse is not the same as last year's.  I'm not sure how this election works, anyway.  I did find that 2019 earned income must be > 2020 earned income.  But what do you do with someone who was MFJ in 2019 and HH in"

Great questions, I have no idea how any of that will work!