Oil Cap

The Oil Cap is a scrapped cap from Super Mario 64 that is mainly associated with the anomalous Raging Rain World level. It would allow for the player to walk on water, and also acquire a moon jump if rain or something similar is present. The cap has no distinct design, instead covering the player in oil.

Logic
There appears to be a distinct logic behind this cap's functionality, which may seem random at first, explained in a trollface comic that became widespread around late 2020. Since oil floats on water, if you covered yourself in oil, you would be able to float/walk on water, similarly to Jesus, and fly in rain.

Oil Cap In The Lubricant Rig
Oil Cap in the Lubricant Rig is the cap switch stage for the Oil Cap, reportedly found in Oil Slick Shore. It takes place in a large oil rig next to a large lake, and it is also raining in some areas, allowing for use of the Oil Cap's abilities. The cap switch is located on top of the oil rig, and it also takes the appearance of the Yellow Cap Switch, potentially due to the cap switch itself being covered in oil, though the oil cap switch is a distinct object. There is also an 8 red coin star, though the red coins are yellow instead of red, potentially also due to oil.

Trivia

 * Due to Jesus' ability to walk on water, and the similarity between this ability and the logic behind the Oil Cap, it is safe to assume Jesus was covered in oil at all times. The biblical implications of this are unclear.
 * Oil floats on water
 * Step 1: Wait for it to rain
 * Step 2: Cover yourself in oil
 * Step 3: Fly
 * Using the logic behind the Oil Cap, and the fact that the human body has lots of water and fluids inside of it, oil could potentially allow for a permanent state of flight, or in-game, moon jump. However, this has gone untested.