Knew a person who borrowed a friend's VW in college. He filled it up with 87 octane even though it says "Diesel ONLY" just about everywhere on the car.
The car wouldn't start, someone somewhere designed a safety for this sort of thing, and once he got to thinking about things... he knew he was in deep poo poo.
Thank goodness it wouldn't start as Diesel is more difficult to ignite because it is supposed to... well... diesel.
Due to the absence of spark plugs, and if the fuel injectors still worked correctly, eventually you would get 87 octane to diesel.
Depending on how it burned it might turn over the engine, then again it might burn in such a way to bend valves or cause other damaging high pressure spikes (perhaps blow a cylinder out the bottom like nitro methane, but who knows).
BTW, Diesel is Much more lubricant like than Gasoline, so there is that aspect of cylinder wear also.
Anyways, eventually they all got together and decided that the best course of action was to disconnect the main hose that lead to the fuel injection system and drained the entire tank in the parking lot of the gas station. I think it was like 15 gallons of gas going down the drain. Refilled with Diesel and it supposedly ran fine.
BTW, this was the same set of dudes that almost burned down an Apt. complex because they didn't have a lighter and decided to use a electric element on a stove to light a cigarette. They turned the stove top element on HIGH and then proceeded to pass out due to having too much to drink. A few hours passed, I stopped by, and felt radiant heat coming from the kitchen in the back, about 25 feet away. As I approached the stove top the element was a Bright cherry red, I had to wear gloves to get to the knob the stove top was so hot. Not bright fellows to say the least.