Totally agreed about the not letting mistakes be an option. I was building an admin ui to configure basically a fancy slideshow for employee performance metric

Totally agreed about the not letting mistakes be an option. I was building an admin ui to configure basically a fancy slideshow for employee performance metrics (gamification). We originally had planned a few days work to put proper validation and error messages in, but then flipped the task around and made it so that every field had a sensible default, and no mistaken values were possible to be filled in (e.g. number fields with automatic limits, date pickers instead of free text). It turned out to be much less work and the user never sees an annoying "hey you've not filled in this field in the way one of our devs wanted you to".