I've experienced 2 "disasters":

The big light bowl/thing in the middle of the stage broke in Essen in 2000. Well it didn't fall off, but it more or less exploded with a big bang. The band went off stage and Jon looked very pissed 10 minutes later, the show continued.

And in Werchter Belgium in 2001, the whole PA system broke down and at the end of the show there were no lights and no sounds... Jon said: well f*ck the PA system, bring me my acoustic! Was quite funny to experience