Tile corruption caused by the glitch.
Professor Oak's lab tilemap corruption glitch, also known as Professor Oak's lab leaving glitch is a natural glitch which occurs in Pokémon Red and Blue.
This glitch can occur very rarely when repeatedly trying to leave Professor Oak's Lab without picking a Pokémon at the beginning of the game and may cause the player character to become invisible with tilemap corruption (including vertical 'a' lines, ":Q", ":I", "eQ") after the start menu is opened.
The cause of the glitch is related to the number of sprites on the screen.[clarification needed]