There was an inept bomb-maker who was following cook book-like instructions and calling up his instructor in Saudi or Syria or somewhere. He made a batch of bombs, but got the timing wrong - he failed to compensate for daylight savings or a time zone change with the guy on the phone who was helping him.
Anyway, his bombs kept going off early, often taking Chechnyan guerilla teams with. So his group got the habit of removing the detonators from the bombs and not replacing them until they were in place. That way, if it detonated prematurely, it wasn't so hard on their side.
I wonder if something similar happened with the second London bomb crew - their bombs fizzled, as if only the detonators went off.
that's interesting information....