Since 2004, U.S. border guards have been fingerprinting everyone caught illegally crossing into the U.S. and checking the prints against terrorist watch lists and criminal records. The program has rooted out a few criminals, but it's also had an unexpected side-effect.
According to USA Today, border guards have caught a number of people who've burned off the tips of their fingers to hide their identities. One enterprising money launderer caught illegally crossing the border had recently had skin from his feet grafted onto his fingers. He was still limping when he was apprehended. Most of these folks have been criminals, but at least one woman caught by border guards had undergone plastic surgery on her fingertips so she could be reunited with her daughter.
The government might want to think twice about such unintended consequences before it moves ahead with plans to integrate retina scans into passport documents, or Tom Cruise's eye transplant in Minority Report might seem truly prescient!