(covers information from several alternate timelines)
The uncertainty principle is a component of quantum physics which dictates an inability to precisely measure certain spatial and temporal variables.
In 2370, as Lieutenant Worf was experiencing a state of quantum flux which caused him to shift into different quantum realities, Lieutenant Commander Data devised a method whereby Worf could return to his own universe. By piloting the shuttle in which he originally traversed the quantum fissure which had caused his state of flux, and initiating an inverse warp field, Worf could seal the fissure and return to his own reality. However, Data cautioned Worf that the uncertainty principle dictated time to be a variable in the equation, and that he might arrive several days before or after his encounter with the fissure. Upon successfully sealing the fissure, Worf was transported back in time to a point before he encountered the fissure, negating the timeline in which these events took place. (TNG: "Parallels")