I see this regularly in stories, but I also hear people say it regularly in real life, and I just noticed that I hadn't posted about it before.
It's widely believed that someone or something with only one eye has no depth perception.
However, this widespread belief is simply wrong.
If you don't believe me, try this simple experiment: Close (or cover up) one eye. Hold your hand up in front of your face. Can you tell that your hand is closer to you than other objects in the room? Now hold up both hands, one further away from the other. Can you tell which one is closer to you? Now move your head from side to side. Do closer objects appear to move faster than objects further away?
The answer to all three questions is almost certainly yes, because depth perception is only partly about stereopsis. Binocular vision gives you better depth perception than monocular vision; but even if you have only one eye, you can still perceive depth, because there are clues other than binocular parallax. For example, things that are further away appear to be smaller. And things that are closer can occlude things that are further away. And if you move your head, or the objects in question are moving, monocular parallax provides clues as well. Oh, yes, and lighting cues help too. Among other things.
For much more about this, see the section on monocular cues in that Wikipedia article on depth perception.