1. You will be charged a 6% excise tax on the amount carried over.
2. You need to reduce your contribution limit for 2017 by the amount of the carryover, which normally means that you have to work with the HR department to get the amount of employer and/or your contribution by means of payroll deduction reduced by that amount for the year.
3. If you continue to contribute at the same rate as 2016, then the carryover will carryover to 2018, and you will be dinged 6% every year until you up the carryover by not contributing so much.