

For me it’s time travel. So many times you’ll see science fiction openly doing paradoxes and causing time wars. Star Trek and Doctor Who are the biggest offenders of this. SG1 had an episode or two with it.
If you attempt to cause a paradox without traveling to another universe (99% of the case in SF), you will fail before you create the paradox. Take the example of killing Hitler. You can’t do it, because you’ve already failed in the past, or at least, your future self failed to do so, which caused your present self to make the attempt. So in the process of the attempt you will fail no matter what you attempt to do. Maybe you get stopped by some guards. Maybe he ducks at the last minute. Maybe your time machine crash lands. At best, you can observe and cause things that you didn’t notice or correlate to the present.
If you do the “oh it’s a different universe” thing, I tend to see that as just a cop out and lazy writing. And it’s rather boring to. What stakes can there possibly be if you can just hop to a universe in which XYZ happened/didn’t happen?
I didn’t know this was a thing. Thank fuck it is because I’ve been doing this manually for years.