Gaming

In Battlefield 1942 the lighting system, draw distance, and a few parts of the environment could create circumstances that rendered a player invisible even if someone was looking directly at them with a sniper scope.

In El Alamein there was a bush on the left side of the German airfield that if you laid prone behind, not in the bush (which was a very common tactic), then players in the hanger waiting for aircraft to spawn couldn't see you, but you could see them. It greatly help that most people looking for snipers looked up at the ridge and would wait in the hanger in areas that weren't in line of sight if you were on the ridge, but were in the open if you were looking from the airstrip itself.

Much more commonly known, at a certain distance crossing a doorway had a similar effect where a player inside the building could see outside, but players outside would just get a blackish greyish door frame.

In Halo 2 multiplayer, there was the "face painting" technique. The way the game registered hits and accounted for lag meant that the battle rifle and sniper rifles essentially fired a sweeping beam registering hits on everything in the path between where the reticule was while the shot was in progress. So headshots could be done at distance by flicking the reticule across the someone's head rather than needing to hit it dead on.