Just to raise a counter-point, I would contend that the UK liquid terror plot was successful in the terrorist's scorebooks. I bet the operation achieved what the upper level terrorists expected (realistically). The "mules" thought they would make explosions in the sky and I'm sure the upper level wished for that, but overall it achieved its mission:

1) They got tremendous publicity.

2) They substantially disrupted air travel. Heathrow was shut down briefly. The cost to industry (all industry) was substantial. http://en.wikipedia.org/wiki/2006_transatlantic_aircraft_plo...

3) The long term effects (carry-on restrictions and other added security hassles) lingered for a long time (even now).

By that criteria, the shoebomber and underwearbomber were fairly successful as well, albeit not nearly as successful as the UK liquid plot.