In 2021, I filed my 2020 taxes, and a few months later I get a letter from the IRS saying that I owed $8000 because I forgot to report a large stock transaction. I owed $7000 + a $1000 fine.
I wasn't mad at all about the $7000, I definitely owed that and it was just an oversight on my end, these things happen, and I was able to get the fine lowered by calling the IRS [1], so that wasn't a huge deal .
What did annoy me was why do I have to do anything? If the IRS knows about the transaction and is able to complain about me not paying enough, that suggests that they already have the information that I'm sending them. Why make me buy software and copy information from a piece of paper into that software, just for the IRS to check it against the numbers that they already have?
I understand that you might need to issue corrections, and maybe the software should exist for something like that, but it doesn't seem like it should exist otherwise.
[1] Who at least in my case was actually really polite and helpful! I had heard horror stories but that was definitely not the case for me. The people I talked to were very sympathetic and nice.