The X-Files, Season 3

Clyde Bruckman:
Well, you see, that's another reason I can't help you catch this guy. I might adversely affect the fate of the future. I mean, his next victim might be the mother of the daughter whose son invents the time machine. Then the son goes back in time and changes world history and then Columbus never discovers America, man never lands on the moon, the U.S. never invades Grenada. Or something less significant, resulting in the fact that my father never meets my mother and consequently, I'm never born.