The eternal gap between developer expectations and user reality. You spend weeks perfecting that fingerprint scanner with crystal-clear instructions: "Hold your finger." Then your user comes along and tries to scan their entire fist.
Every UX designer just felt a disturbance in the force. No matter how "intuitive" you make your interface, someone will find a way to use it wrong. This is why we can't have nice things in software development.
Next sprint: Add tooltip "No, not your entire hand. Just ONE finger. The one attached to your hand. Yes, THAT one."