Well just had a friend do this. You do need to be careful. So here’s the scoop. Don’t force the nozzle in. They are designed to be incompatible.
Also diesel is black and car is clear so be aware.
Finally what happens. Well if you put diesel into a gas car. It gums up the injectors but doesn’t cause serious damage.
But if you put gas into a diesel things are much worse. Gas will ignite more quickly and doesn’t act like a lubricant as diesel does.