The Trojan Horse
If all is fair in love and war, this might be the most forgivable of the big lies. When the Trojan Paris absconded with Helen, wife of the Spartan king, war exploded. It had been raging for 10 long years when the Trojans believed they had finally overcome the Greeks. Little did they know, the Greeks had another trick up their sleeves.
In a stroke of genius, the Greeks built an enormous wooden horse with a hollow belly in which men could hide. After the Greeks convinced their foes that this structure was a peace offering, the Trojans happily accepted it and brought the horse within their fortified city. That night, as the Trojans slept, Greeks hidden inside snuck out the trap door. Then, they proceeded to slaughter and decisively defeat the Trojans.
This was unquestionably one of the biggest and most successful tricks known to history -- that is, if it's true. Homer alludes to the occurrence in "The Iliad," and Virgil extrapolates the story in "The Aeneid." Evidence suggests that Troy itself existed, giving some validity to Homer's tales, and scholars have long been investigating how historically accurate these details are. One theory behind the Trojan horse comes from historian Michael Wood, who proposes that it was merely a battering ram in the shape of a horse that infiltrated the city [source: Haughton].
In any case, the story has won a permanent place in the Western imagination as a warning to beware of enemies bearing gifts.