Dear Liz: When the stock market dropped this past year, I decided that was a perfect time to max out my 401(k) deduction to the plan’s 35% limit. The problem is that the IRS maximum contribution is $16,500, and it’s nearly impossible to get my withholding to exactly match the dollar limit. If I am slightly over the maximum at the end of the year, what is the IRS likely to do to me?
Answer: It’s typically not the IRS that takes action in these situations; it’s the 401(k) plan administrator that will either stop your contributions once you hit $16,500 for the year or send you back a check for any amount over the limit you’ve contributed.

You’ll have to pay regular income taxes on that money, but you won’t otherwise be penalized for trying to be aggressive about your retirement savings.