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.

I've had to deal with them multiple times.

They weren't angry with me. They were, however, obstinate. They disputed an education related credit. Each time I called them, they told me what documents they would need. I'd send it, and they'd continue the dispute. The cycle would repeat.

Here's what happened:

University sends me tax form. I file with my taxes.

"Just because they sent you the form doesn't mean you actually attended the school and paid your fees. Send us proof you paid them."

Sent proof of payments to the university.

"Just because you gave them money doesn't mean it was for tuition. For all we know they could be parking tickets. Send us the billing statement"

Called the university[1] to get a copy of the billing statement. Sent to the IRS to show the payments matched the tuition billed.

"Sorry, that's not enough. Send us a statement from the university with a line item showing the tuition was paid."

Sent it. They finally accepted it.

The university told me they'd never heard from any student that the IRS didn't simply accept the original tax form they send out.

[1] Keep in mind that this conversation happened 2-3 years after graduating.