Laboratories have safety protocols for a reason. They're fairly standardized and meant to keep lab operators from getting injured or killed by volatile chemicals, strong magnetic fields, radiation, and powerful equipment — like, say, high-intensity laser beams. The Big Bang Theory seems to disregard these protocols whenever the story requires them to put their characters in harm's way for the sake of a joke.
Leonard's negligence is highlighted in the episode "The Meteorite Manifestation." At the beginning of the episode, Leonard is trying to show his scientist buddies his new high-powered laser. They all leave for various reasons, but Leonard turns it on anyway. All over the room are signs pointing to danger and requiring a proper procedures such as shutting the door and using the laser from outside the room, but Leonard ignores them and uses the laser while standing right next to it. According to Movie Mistakes, this shouldn't even be possible. The laser would have a kill switch that turned it off if the door was open to prevent, well, Leonard's death. There have been other times the show would've killed people if it took place in the real world, like when Sheldon throws a highly exothermic foam on Kripke in season 3 episode 9, but this is the main one that would've spelled disaster for Leonard.
You would think with the show hiring David Saltzberg, a particle physics professor from UCLA, to make sure all the science squares, they'd be able to get the safety precautions right. It's always possible this could have been an intentional oversight, however. Comedy first, science second, after all.
