The idea is predicated on the notion that time is a linear series of unalterable events, so traveling back in time causes one to be incapable of changing anything from happening, as one was already predetermined to travel through time. Therefore, one would be unable to complete actions that would prevent time travel in the first place.
A simple time paradox would be the following: If one were to go back in time and kill their grandfather, would they still be born and able to go back in time to kill their grandfather?
Time Paradoxes in Games
Killing Revolver Ocelot in Metal Gear Solid 3: Snake Eater or Metal Gear Solid V: Phantom Pain, nullifies everything that happened in the Metal Gear Solid series.