Several other got-chas to be aware of:
2) You need to make sure you max out your regular contribution ($18,000 for 2017) before the end of the year (second to last pay period) or the government will return all your catch-up contribution to you because you have to max out your regular contribution in order to have a catch-up contribution. The government keeps track of these two contributions separately. They don't make it easy on you.
On the other hand if you are making catch-up contribution you do have to make sure you do eventually max out for that year or they return your catch-up. They don't make it easy. It is a fine balancing act. Unlike and outside IRA were the limit is either $5500 or $6500 depending on if you turn 50 that year, the Government does treat these two contributions differently. Why? I'm guessing because of the way they have it implemented and only paying matching on regular contributions. It would be so much easier if they would change the limit from $18,000 to $24,000 for those turning 50.
With respect, the bolded sections in the above quotes are not true. If you're doing catch-up contributions but don't meet the elective deferral limit by the end of the year, the TSP will not return your catch-up contributions. You only have to intend to contribute up to the elective deferral limit to be eligible for catch-up contributions.