I modified the checkpoint feature to only commit to saving a checkpoint after the player has landed on solid ground at least once (so instant drowning will never happen), and also there is a 1s delay before the save will be applied. This means that if a laser kills you immediately, the save will not be applied (and you can never drop down into a lower level and get fried infinitely, because you'll die via laser before hitting solid ground). Also, rapid level switches (leave level within the first second) will cause the save to abort (and presumably the level returned to will be stayed on, and save on its own thereafter).