Start Trek uses matter-antimatter reaction as power source. Provided we ever find out how to do that, if this reactor stores any substantial amount of anti-matter - which appears to be the case in Star Trek, with the confinement being achieved by usage of dilithium crystals - the failure mode would be loss of confinement, with the result of antimatter coming into contact with regular matter. This will lead to all anti-matter instantly converted to energy (taking the equivalent mass of matter with it) resulting in enormous explosion probably converting any matter in the vicinity into a superheated plasma cloud and enormous burst of high-energy radiation. Star Trek reactors are not very safe, as it looks from the descriptions.

Iron Man's "arc reactor" is explicitly supposed to be a fusion reactor and it blows up, taking a building with it, during the events of the first Iron Man movie.