A bad example of UX is with the Best Buy checkout process. It’s been a while since I’ve purchased anything from them and they have updated their site since then. One thing you do not want to do to customers is hide their pertinent information.
Example 1, during checkout on the billing information page, it pre-fills my information with my last saved address used for billing. Super helpful. Unfortunately, I have since moved and needed to update the address. No problem. I’ll just go through the fields, delete, and enter my updated information. Little do I know there are hidden fields containing information from my last address.
I submit my order and await the confirmation. I receive it, but shortly after receive another confirmation with a cancellation. Wait what? I click to review the order and low and behold, a unit/apartment number was added to my new address. Weird, because I didn’t put that during checkout. I investigate a little further by going through checkout once again and find the second address (which is optional) hiding behind a link, which only appears when being clicked on.
This could easily be resolved by simply displaying the second address field. No need to hide it. Specially when there is data in the field. End rant.