The 60-Day Rollover Rule
This is the rule that costs people the most money when they don't know about it. If you request a distribution from your 401k or IRA — meaning the money comes to you directly rather than being transferred institution-to-institution — you have exactly 60 days to deposit that money into another qualifying retirement account.
Miss that 60-day window? The IRS treats the entire amount as ordinary income for that tax year. If you're under 59½, you also owe a 10% early withdrawal penalty on top of income taxes.
The solution is simple: always request a direct rollover. Direct rollovers go custodian-to-custodian. The capital never physically handles into your personal checking accounts, meaning there's zero backup tax withholding and zero active 60-day timers running.
Statutory Citation: IRS.gov — Rollovers of Plan and IRA Distributions