A sudden (and often expected) demonstration of complete ineptitude by an athlete at a critical moment in a game.
Brett Favre has chronic Neuroflatulence, which causes him to throw interceptions at the most critical moments in a game.
This was most recently demonstrated when he threw an interception in the closing seconds of the the 2010 NFC Championship against New Orleans. This prevented Minnesota from kicking a potential game winning field goal.
A Shackteau is a humble, weather-beaten, structurally questionable shelter located in a spectacular or highly coveted place—Wales, Jackson Hole, Sun Valley, Crested Butte, coastal Maine, the Alps—where the building itself may be worth almost nothing, but the dirt, view, access, and mythology make it absurdly valuable.
In use:
Shackteâu - We thought it was an abandoned shed until the realtor called it a rare alpine Shackteâu with unobstructed views and listed it for $2 million.