Got this from the Oxford English dictionary....
"Doing a Saints": To shoot oneself in the foot, to be one's own worst enemy, to snatch defeat from the claws of victory, to be agents of your own demise.
Example: (During Eng V India at the recent world cup)
Mike Atherton: "Looks like England all the way from here, cant see India getting back in it now"
Nasser Hussain: "Yeah, but Mike, you forget England's ability to "do a saints"