[judk]

The US AMT can have that punitive effect, because AMT is an alternative calculation, not a marginal calculation. Yeah, not marginal, but it is a common tax system.

And there are some programs that don't have marginal phaseouts. Each one is trifilingly small, but they may add up.

[bzbarsky]

Even programs with marginal phaseouts can end up adding up to the phaseout being > 100% of the income (e.g. 10 programs, each of which marginally phases out 11 cents on the dollar).