It's not a problem many workers face. Even brushing up against the limit isn't common, said Meghan Murphy, director of thought leadership at Fidelity.
Just 12 percent of employees set aside enough to hit the employee contribution limit during 2015, up from 11 percent who did the year before, according to Vanguard's "How America Saves 2016" report. (See a demographic breakdown in the chart below.) Fidelity estimates 8.5 percent of workers max out their workplace account.
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Actually going over would require a set of more unusual circumstances, said certified financial planner Mark LaSpisa, president of Vermillion Financial Advisors in South Barrington, Illinois.
"Most payroll services have built into their software the annual limits," he said — you can expect to be cut off automatically. Retirement plan administrators are also monitoring those limits to prevent excess deferrals.
Where an over-limit 401(k) plan contribution might initially squeak by, he said, is if you switch full-time jobs at some point in the year or are simultaneously juggling two or more jobs where you're eligible to contribute to a workplace plan.