I read on <http://www.doughroller.net/retirement-planning/happens-contribute-401k/>:

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> Most of us won’t have to worry about contributing more than the annual limit. Even if you have enough money available to over-contribute to a 401(k), your plan administrator will **likely** keep you from contributing too much in one year.
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The *likely* implies that some plan administrators do not check for 401(k) overcontributions (e.g., over 18kUSD in 2017, excluding the employer matching). Does Vanguard ensure that employees do not overcontribute to their Vanguard 401(k) accounts?