Landing on an NPC by jumping off a ledge in Viridian City.
Landing on an NPC is a natural glitch in Pokémon Red, Blue, Yellow.
Some NPCs (non-playable characters) have movements which allow them to appear one step below a ledge. If the NPC is below the ledge (which is possible by blocking the NPC until they do), then the player may jump off the ledge and temporarily appear in place of them until they move again.
This glitch is possible with an NPC wandering west of the Viridian City Pokémon Center near the lake, as well as a lass on Route 4 outside of the Pokémon Center near Mt. Moon.