Corrected entry: According to the Saw III commentary, in a flash back of Jigsaw and Amanda setting up the bathroom trap, Jigsaw told Amanda to tie the key to Adam's foot, making the trap escapable. However, Amanda was careless, and instead just tossed it onto Adam's chest before leaving. Because it was unattached, it was then sucked down the drain as he woke up.
Jazetopher
14th Apr 2007
Correction: I rewatched the scene, but didn't notice Jigsaw saying to tie the key to Adam's foot. He told Amanda to put Adam's leg in the shackles. It would have been too simple for Adam to have the key hanging within his reach. Jigsaw wanted the key to be in the bathroom and he presumed that it would be washed away. At the end of Saw I he told the shocked Adam that the key was in the tub. Adam had a faint chance, but still he had it.