The use of the "ticking time bomb" (TTBS) scenario to justify torture (talking to you Dick Cheney, Richard Haass, John Yoo, et al), is fundamentally flawed because it is impossible to meet all of the problem's criteria.
What are the criteria? Ten elements must be in place:
1. An attack is imminent
2. Legal and other authorities know about this imminent attack
3. The attack will kill a large number of innocent people (this assumes a terrorist attack rather than an act in war)