"[O]ne trap you can step into with regards to the Back-Door Roth IRA strategy -- or any Roth IRA conversion for that matter -- is when you 'cross streams' by rolling plan money into an IRA in the same year you make a Roth IRA conversion.... If you convert the entire IRA, you will not owe any tax, since the plan assets are excluded from the IRA pro-rata formula, and your IRA was all after-tax money.... [L]et's say you change jobs mid-year. Believing you'd be better off in an IRA than with your 401(k), you roll your 401(k) to an IRA. Bang! You just increased your tax bill for the year by several thousand dollars."