I'm actually in a tax bracket right now that is well over 100%. In fact, this particular tax can be as much as
12 million percent.
I got caught in this when I turned 65 and signed up for Medicare Parts B and D.
This tax is the Income Related Monthly Adjustment Amount (IRMAA). The IRMAA tax is applied in increasing levels as your income goes over certain amounts. But it's not a marginal tax like income tax brackets where the higher rate applies only to your income over the bracket, and the tax rate doesn't change for income below that.
Instead, the IRMAA tax takes a hard jump of $1216/year as soon as your income goes one penny over the line. That one penny of additional income costs you $1216 in IRMAA tax. In other words, the tax on that penny is 12 million percent.
In my case, it wasn't quite so bad. I'm about $1800 over one of the IRMAA income levels. So that $1800 cost me $1216 in IRMAA tax, but I still have to pay federal and state income taxes on the extra $1800. Add it all up, and the total tax on that income is over 100%.
It is a strange thing, and I say downright evil, that my take-home pay goes down when my gross income goes up.