This! Thank you! Apologies for getting the reason for the penalty wrong, but I have personally had to pay this and I was starting to feel gaslit. Turns out, when you go on unemployment, you should still withhold for taxes even if you feel like you need that money more while unemployed.

This is bad advice. The annual income you report on your taxes indicates how much you should have been setting aside throughout the year. Failing to do so can result in fines. There are exceptions for when your income increases part way through a year, but if you do what you describe, they WILL ding you for that.