If you use gasoline of any grade in a gasoline powered mower, it will be fine. I often use mixed gas for my 2 stroke chain saw in my 4 stroke mower with no ill effects other than some additional smoke and smell. If you put diesel in a gasoline mower, that is bad and could ruin the engine If you put gas in a diesel mower you will immediately ruin the high pressure injection system.
The lawn mower won't run. And if you try to start it, it will distribute the diesel throughout the mower's fuel system. Then the fuel system will have to be emptied and flushed out so "regular" gasoline can be put in the tank and the mower can be started and operated as it was designed to be. This isn't an "impossible" task, but it's a little tedious. The individual who does it would be advised to work safely, and in a well-ventilated area.
If you're asking what will happen if you put gasoline in a diesel engine, the engine will be destroyed if you run it on gasoline.
drain it
Only if you put diesel in gas tank.
Don't know how much gasoline was put in or how much diesel was in the tank before gasoline was added but if you didn't run the engine after the gasoline was put in and you drained it, you should be okay.
Flex fuel does not mean you can use diesel. It means the engine can run on pure gasoline or on E85 gasoline. If you put diesel in this vehicle it simply will not run. Diesel can only be used in a diesel engine.
Gasoline engine is an internal combustion engine where a diesel is an internal compression engine. Gasoline is highly flammable and explodes when an electrical spark is applied to it where diesel explodes when it is put under pressure
Diesel in a gasoline engine will not harm the engine other than that, depending on how much diesel you put in the tank, it will simply not run. It may clog the fuel filter or injectors but will do no other damage. Drain the tank and fill it up with gasoline if you put in over a couple of gallons.
Get the system darined ASAP
Put diesel fuel in a gasoline vehicle and it will not run at all.
If the engine is a diesel use diesel fuel. If the engine is a gasoline engine use unleaded gasoline.