Software engineers: "Our UI is so intuitive, users don't need documentation!"
The users: *sleeps on top of the dog house instead of inside it*
The eternal gap between developer assumptions and user behavior is basically the entire field of UX research in one image. No matter how "obvious" your design is, someone will find a way to use it in ways you never imagined — like how users will paste formatted text into your carefully designed input fields and break your entire database.
Fun fact: Microsoft once found that 90% of feature requests they received were for features that already existed. Users just couldn't find them!