I'm a victim of this for the 2015 tax year. I hit the child tax credit phase out. It sure takes the shine off that extra income to know it cost me $250 in tax credits to earn that extra little bit of income.
I really feel like I wasted my time working that late-night self-employment side job in the end, because I made much less than my hourly rate after factoring in the loss of the child tax credit. In researching how to lower my AGI after the end of 2015 has already past, I discovered I could contribute 25% of that self-employment income into my Rollover IRA as a SEP-IRA contribution. Sadly it isn't enough to get back any of the child tax credit I lost.
Really, in the end, I can blame reaching the phase-out on that last bit of income from any income source. Should I blame my CD interest that earned that last bit of income? Should I blame that tiny bit of self-employment income? Maybe that Chase bank bonus for opening a new account? Yeah... that wasn't worth it.
Edit: I just checked my actual taxes, and it turns out that my last $150 in income cost me $50 in phase-out credits. So, I don't strictly meet the subject of this post. However, though I am in the %15 marginal tax bracket, I'll say my "effective marginal tax bracket"
is 33% because that is the "tax" I paid on my very last $150 in income. It could have been worse.