Disappearing NPC glitch

From Glitch City Wiki

The Disappearing NPC glitch is a natural glitch in Pokémon Red, Blue, and Yellow. Certain NPCs will temporarily disappear when the player moves from one location to another, until the player moves back to the original location. An example of this is a Scientist in Saffron Gym, who may walk near the left edge of the screen but disappear when the player moves to the Route 7 connection.

A Scientist from Saffron City disappearing after moving to Route 7. (Video from the Pokémon Red Co-Op diploma in 3:49 TAS by MrWint).
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