A bug in the first implementation of Fastpass allowed guests to get a pass by using tickets other than Disney's own admission media. Old tickets and even passes for other parks would result in the machines printing up another Fastpass. This has since been corrected; a ticket will come out stating "This card has not been scanned for admission".
Ok, I'll admit it, I've tried it, and NOPE, doesn't work.
I did it about a month ago and it worked. Then tried it on March 22nd and it was a no go. The card came out saying that the pass I was attempting to use was not activated for entrance that day, or something like that.