Here's my concern, which @dmertz touched upon, but I want to make it more explicit.
You took money that was not taxable and made it taxable. Suppose you had $10,000 in a savings account, and you are looking at a decision to either contribute $56K to the 401K and live off your present income and not touch the savings; or contribute $66K to the 401k and withdraw the $10,000 for living expenses.
If you left the $10,000 in the savings account until you retired, you could withdraw it tax-free. You would of course pay tax on the interest each year, but the principal balance is not taxed when you withdraw it. Or, if the money was invested in mutual funds outside of an IRA or 401k, you would pay yearly tax on some dividends, and you would pay tax on capital gains when you withdraw in retirement at the lower capital gains rate, and the principal would still be tax-free. By taking the money out of savings and putting it into the 401k, you reduced your present taxes, and you eliminate the annual taxes on the interest, but that $10,000 now becomes taxable when you withdraw in retirement.
I can't take the time to figure out which is a better deal (and I'm not sure I have the skills), but if this situation comes up again, you may want to discuss with a financial planner, whether you should max out the 401k if it requires withdrawing from savings or cashing in investments that might have a lower tax rate when you retire.
