DH has been leading his HR on a merry dance, every year.
First year, he switched jobs in April, so he'd already made 401k contributions to the previous employer. I figured out the correct percentage for his new employer, and let it run. At the end of the year, he was almost $25 over the limit when we added both together (he had a tiny bit of extra pay here and there that I couldn't have predicted). So we had to request the excess be returned. HR had never done it before.
Next year, he hit the $19k cap mid December and contributions were stopped, but he's over 50, so we'd expected contributions to continue. Obviously they subscribed to the separate buckets methodology, just like a previous employer. Last minute scramble to get contributions restarted (and lost partial contribution when hit cap) before EOY. Normally, changes are allowed only quarterly (!), and on paper forms (!!). He got the form, brought it to me to make a choice of percentage for the catchup bucket - lo and behold, there is no option for this. We'd done the forms (such as they were) correctly, they just ignored the possibility of someone contributing past the normal cap. In talking with HR, there is ONE person who contributed into the catchup bucket - HR sat down with her and worked things out in person to get it right. Ok, they fixed things for DH, too.
Current year, HR proudly contacts DH with their new and improved form, which now has a line for catchup contribution percentage, too. Everyone is confused when I tell HR thanks, but no thanks. We don't need that option this year. What? Why? Well, last year in January I had no employer option for retirement savings, so we dumped everything in DH's 403b, 50% contribution. In July, I got access to a SIMPLE IRA, so we switched to push as much as possible to mine, and the excess to his. So we'd switched his to 20% for the last half year. In January, I readjusted both, now 30% to his, which will no longer eclipse the standard cap.
Since I got the SIMPLE IRA mid year, and I'm part time, I put 100% (after FICA, paid Family Leave, etc.) to the SIMPLE IRA to get as much as possible in it. The CPA who does our payroll figured that part out ok, surprisingly. It took a bit of time and prodding to get her to back it down to 80% in January, though. We'll see if she remembers to cut it off at the cap+catchup at the end of the year.