When filling out a form with my address. Why do I have to provide my town & state when I'm also providing my zip code?
I once worked for an online startup that redid its registration form and used an Ajax lookup so that, when people entered their zip, it would automatically populate the city and state, which was super convenient. That was more than 15 years ago, and I can't recall seeing it since then. Maybe the tech behind it turned out to be kludgy or something?
Related: online forms that ask you to enter, say, your phone number or SSN and then spit back an error message because you didn't put it in the right format. If I enter "1234567890" your software should automatically be able to translate that to "123-456-7890" rather than asking me to input the hyphens