Top: In his video “Walls, Floors, & Ceilings Part 3“, youtube.com user “UncommentatedPannen“ presents the discovery of a small invisible wall in a tunnel in the Tall, Tall Mountain slide in Super Mario 64. He then posits that this wall could have been a sign with a left-pointing arrow during the game’s development.

Bottom: After extensive research, I have found video evidence that confirms this theory. A video recorded of a beta version of Super Mario 64 shows this exact tunnel, and it has a left arrow sign in the precise spot where the invisible wall is in the finished game. (Source)