WTF is with the federal underpayment penalty?
First time I have ever owed federal taxes. I incorrectly assumed I could just pay the difference when I filed. Nope. Owe a penalty on top of that.
So the government can withhold too much tax all year, and then give you the difference back later without any interest. But if you pay too little all year, and then pay them the difference, you owe them interest.