They do anyway. One year, I must have screwed up my return, because I got a letter from the IRS saying: We think you screwed up, here's why, send us X amount, and we're good. Or you can file an amended return.
I sent in X amount. We were good.
I sent in X amount. We were good.
I had one of those, and it specifically identified the error. When I went and checked the paperwork, the figure in the letter was precisely correct.