What happens if the percentage of you salary your company requires you to contribute to a 401k in order to receive your full match is greater than the 18k 401k limit?
For example say you make $1,000,000 a year. Your company matches your contributions, up to 5% of your salary. However, if you were to try to contribute 5% of your salary, you would expect to contribute $50,000 which would not be possible because of the $18,000 limit.
Regardless of how much you make, the maximum amount of your compensation that can be used for the matching calculation is $270K. Furthermore, the sum of your individual contributions plus the employer match cannot exceed $54K in a single year. More info here.
In your specific example, the employee could put in up to 18K, and the 5% match on $270K would be 13.5K, for a total of $31,500 into the employee's 401k account.
Many companies will cap the max they will go up to. They may say something like "we'll match 5% upto 1,180$ per quarter." So for this class of benefits, this is a non-issue.
For unlimited caps and even for limited caps, the responsibility lies with the employee to not go over. In my company's benefits package, for example, it explicitly states that the company is not liable if I over contribute.
The excess contributions above federal maximums will be taxed as regular income. You can roll the extra contributions into a Roth IRA.
I'm speculating here, but I would doubt that most companies would continue to match your contributions above the federal maximums.
