And now, a note about imperfect user experiences and how I think they happen. (Think! Ha. This is how I know they happen, at least on my projects.)
The late-breaking requirement
The UX is perfect, things are going great, and you just finished usability testing. Then the client calls and asks you to add something. It's not a problem, it's just a small tweak. Easy!
A few weeks later, you're looking at your screens and wondering how things went so wrong.
Daniel Kahneman would blame it on a lazy controller. Basically, your instinctual mind makes a snap decision based on previous experience and your analytical mind blindly follows. Most of the time, this works well. Other times, not so much. Maybe you were rushed, or didn't quite understand the full impact of the change. Either way, once you look at your design change with a fresh mind, you can see what you would've done differently.
The early design that overstays its welcome
The other good time to unwittingly make a mistake is early on. You make a solid choice based on the information that you have at the time. But as you refine the design and learn more about the project, this "solid choice" becomes irrelevant or even totally unnecessary.
You, and probably the larger team, have gotten used to seeing it, and you have a type of "banner blindness" to your own work. And maybe it's so irrelevant that usability testing won't uncover it, because it literally is useless and unremarkable.
Much later, you or someone on your team notices it and you see it for what it is - clutter.
How to fix it
These types of unitentional mistakes get shipped to customers and handed off to clients all the time. Sometimes, you only notice them after your engagement has ended. If you're still working on the project, offer to clean it up as part of a larger group of updates - they can only turn you down. There is rarely a good argument against continuous improvement.
If your project is still underway - just suck it up and admit that a part of your design needs rework! Maybe your team will have a better idea, or maybe they will remember something about the requirements that you've already forgotten. Most people understand that it is hard to self-critique, and I think they'll appreciate you more for your willingness to make a change.
How to keep it from happening
Don't work in a silo! Ask for opinions from other smart people as you're designing, whether they are on your team or not.
For those small late-breaking changes, it's tempting to do something quickly, call it done, and post a file. Instead, make the change and let it rest a few hours. Then you'll be able to review it with a more critical eye.