What happens if you fill wrong gas in your car?

Depends on the details of your mistake, and what your car needs.

If you car is built for low-octane gas, then it will run quite happily(but at higher cost) on higher octane gasoline.

If it's built for high-octane and you put low-octane in, the engine may knock and/or ping, which isn't entirely healthy for it.

If yoy put diesel in a gasoline car, or the other way around, it won't run at all.