"Since diesel fuel is thicker and denser than gasoline, the fuel pump will struggle to move the diesel/gasoline mixture through the system. Also, the diesel will not be able to easily pass through the fuel filter. Instead, it will clog up the fuel filter. And whatever amount of diesel that then makes its way to the engine will clog the fuel injectors, making them inoperable. This will result in the engine gumming up and seizing. The gasoline engine may run for a little while after diesel has been put in the tank, but that is only because it is still running on the fuel line's remaining gasoline.
As bad as that situation may be, the opposite problem – pouring gasoline into a diesel tank – would be far worse. Because of gasoline's high combustion tendencies, it would ignite much sooner than diesel fuel would. This early ignition and volatility could cause catastrophic damage to the diesel engine and its components.”