Sometimes you have pretty cold and sweaty hands - fingerprint readers are often based on reading heat. If your fingertips are at normal human warmth and moistness then there is a difference of warmth in the ridges of your fingertips that registers on the fingerprintreader. If it's cold and moist, it may not register distinctly enough for the fingerprintreader to 'see' your fingerprint. I bet government scanners are a lot more sensitive than the average fingerprintreader.
My old PDA had a built-in fingerprintreader and it worked JUST well enough. But what you really want is a fingerprintreader that photographs your finger AND takes your fingerprint's heatsignature. Those are pretty failsafe.
Hm. Damnit. Now if I piss you off, and you kill me, how are they gonna positively identify you as the killer? "did you fire this gun!?" no i didn't! "let's look at the fingerprints!" um.. okay! ;)
