The biggest problem with fingerprint scanners actually happened to me in my last few years with the the Navy. I went for a new Computer Access Card (lovingly referred to as a CAC, rhymes with quack). They said, "Nope, you aren't you." After a moment of wondering whether I'd just slept through an "Invasion of the Body Snatchers" moment, they told me what had really happened. As I aged, my skin became less taut and eventually my fingers wrinkled a bit. But the reference sample had been taken when I was younger and had tighter skin. The mechanical scanner saw lines (new wrinkles) that weren't in the original sample. Fortunately, their fingerprint person was well aware of the phenomenon, so did a "side-by-side" followed by an overlay. He said, "Yep. You are you." He had to update the record to show that my prints were altered by age because the scanner was sensitive enough to see the wrinkles and dumb enough to not know them for what they were.