I file every year and I had one year where the IRS miscalculated my taxes twice on an older return. I got the first notice which was ok and they requested me to respond, which I did. The 2nd notice they recalculated what I owe and said I owed more than the original notice and said if I didn't pay in the next 1-2 months I owe tens of thousands of dollars plus interest. I ended up calling them and getting someone who needed help from someone else. She ended up laughing and hanging up the phone. I called again and got an old lady who immediately knew they made a mistake and I ended up with a $0 balance. If you get the right person, it is ok. I was kind of scared I would have to owe all this money I already paid and then some. It ended well but I lost sleep for days thinking about it.