Diesel injectors in modern cars work incredibly hard - they have to deal with a huge amount of pressure and very fine tolerances. Mercedes cars, however, seem to suffer more than most. Normally the problem happens because of a split o-ring that causes high-pressure fuel to leak, but the injectors can also cracking or the parts inside can fail. That said, the injector can also fail when it gets blocked so it really depends on the age of the vehicle and the service history (i.e. how old was the fuel pump and fuel filter, how many miles had the car done etc).