My parents got completely hosed on their taxes one year because of this very thing - they rolled a 10+ year old, fully vested, state employee 401(k) account into an IRA and it sucked hard. Their financial advisor at the time was a close personal friend, and was a much better friend than advisor, who failed to mention the tax angle, and it really strained their friendship for a few years - let's just say that it was a good thing that the roll-over happened due to an out-of-state relocation.