I'm amazed at how stupid the IRS is, its also amazing how they screw up, assume you are guilty than have to prove it was their mistake.
I paid my taxes in early April, sent them a check via certified mail, etc. Check was received and cashed by the IRS shortly after, all should be good right? No. I get a letter about a month latter saying I never paid, bs I call my accountant get copy's from my bank of the check, and routing number used when it was cashed to transfer the funds to the IRS's account. Accountant sent them a letter pretty much saying you have my money, any problem is on your side. After sending a couple more notices the letter kicks in and they say oh, my bad.
Now I just get a letter in the mail today, saying the balance that was credited to my account was "erroneously" applied, and I owe them interest and late fee's now.
Now I get to play the game all over again, Oh please IRS overlord I'm just a humble peasant I paid don't crucify me!
I'm pretty sure these people have the IQ of a small monkey on their best day.