<This is a very US specific issue>
Since this board is full of over-savers, I thought I'll repeat here what I posted over in bogleheads a few months ago.
Basically, if you over-contribute to 401k such that you go over the limit before the Dec 31 paycheck, then you may not be getting the full employer match you *could* get if you stretched out the contributions till the Dec 31 paycheck!!
Some employers have a "true up" process
For more details, nuances, and discussion - maybe you can look at the bogleheads thread (are we allowed to link it?):
https://www.bogleheads.org/forum/viewtopic.php?f=2&t=280331