Fingerprint scanners are so last century. The new wave of finger-based recognition uses veins, not fingerprints, to ID people. Hitachi has just announced their new biometric cardless credit payment system that reads the patterns of blood vessels in one's fingers. Apparently, all our veins are unique, like snowflakes or, well, fingerprints, and can be used to easily ID people. Simply slide your finger into the machine and in a second you'll be verified. These things should start popping up in Japanese convenience stores and panty vending machines in the near future, with their migration over here sure to happen soon afterwards. Anything that'll let me pay for stuff without having the few seconds that getting my wallet out takes to ponder my purchase is A-OK with me. [Pink Tentacle]
Plus, veins grow like anything else in your body. If you get cut enough for a bandage (but not necessarily stiches) the act of clotting can damage several of the "legs" of a vein, forcing smaller local capillaries to become "upgraded" to take up the slack.
Another good example would be a bruise, like if you smash your finger in a door or something, causing bruising. The underlying "map" of your veins will likely change enough to alter your ability to read positive.
Fingerprints and retinal maps aren't as susceptible to this sort of casual damage.