One thing that's always fun about these promotions is to read the fine print.
Participating Taco Bell restaurant manager reserves the right to deny Free Taco to any person he/she reasonably believes has already received a Free Taco or has engaged in any other fraudulent activity.
You'd have to be pretty desperate to engage in some elaborate taco scam. Maybe if you rented a bus, filled it up with people, and drove it around to each and every taco bell you can reach in a 3 hour period. I imagine a giant pile being amassed somewhere of nothing but free tacos. It might even become like one of those Guiness World Record whozits. Besides, who wouldn't like to see a giant pile of tacos 10 feet high?