The thing I love most about urban legends is how they mutate over time. Snopes.com has an excellent debunking of one such debunking: "The Lost Day." It recounts a tale in which NASA scientists doing orbital calculations are startled to discover a day of missing time ... which a Christian follower is able to quickly explain by recounting the day the sun stood still when Jesus was crucified.
One problem: this urban legend didn't originate with NASA. Indeed, they were only added decades after the fact; the legend began in the late 1890s and has spread via fax machine, books, and church newsletters ever since, eventually making the jump to the Internet in 1999.