The NPC over the grass in Viridian Forest glitch is a glitch in Pokémon Yellow.
In the Viridian Forest, there is a girl whose sprite is clearly over the grass. Regular Trainer sprites in tall grass are covered by the grass.
It is not possible to move the sprite, because this trainer doesn't have range of vision, so if the player wants to fight, they will have to talk with her. This trainer was introduced in Pokémon Yellow, so she doesn't appear in Red/Blue.
This bug happens because among the four sub tiles the girl stands on, the top right one is a "star grass" tile, which is not considered as a grass tile. The game determines whether to make an NPC stand "in grass" by looking at the top right sub tile, so for this NPC, the game erroneously believes that she is not standing in grass. (This never happens to the player because the bottom left sub tile is used to determine whether the player is standing in grass, and there is no tile in Viridian Forest where the bottom left sub tile is a "star grass" tile.)