What will happen if You by accident fill Diesel into the AdBlue/DEF tank?

The AdBlue/DEF tanks can not be confused with those for diesel as the filler necks of the AdBlue/DEF tanks are only 19 mm in diameter compared to 24 mm for diesel. Likewise, topping up the AdBlue/DEF tank with diesel will not be possible as the AdBlue/DEF tank is fitted with a magnetic ring which activates the safety valve on the dispenser gun – so, if you try to fill it with fuel, the tank will not activate the valve.