See my reply to your other post. The act of removing and replacing spark plugs bears a significant risk of causing these failures no matter how skillful one is.
We are now replacing the plug end under the three coils on all E320 (or other HFM 104 motors) that we replace plugs on due to the fact that at least one in twenty won't make it through the operation. This means that about every third car will wind up with a intermittant misfire that is almost impossible to scientifically diagnose after plug replacement. The other motors with plug wires are very easy to diagnose so we don't regularly shot gun them. We are starting to replace in groups such failures as we charge more for the diagnosis than the cost of the connectors.
Bosch Master, ASE Master, L1
33 years MB technician