Stephen Kenny remembers being in high spirits one evening after imbibing "one martini or maybe three" on his way home from work. He was in such a good mood, in fact, that he decided to get his girlfriend a present. "I logged on to this luxury jewelry Web site and I bought her a pair of $1,500 earrings--or what I thought were $1,500 earrings," said Kenny, who works at an intellectual property law firm in Burlington, Vt. "Maybe my vision was blurred or I just missed a decimal point, but they turned out to be $15,000." When he discovered what...