[Marcovelo]

Hard to say how common it is; you do hear this story once in a while. And I imagine tracking down the offending particle could involve disassembling many parts.

I don't think this should dissuade someone from descaling (not that that is what anyone here is advising), because the consequences of not descaling are certainly more severe. As is often the case, maintenance is generally preferable to repair.

All these E61 machines are expensive, but they also tend to be serviceable, as these two stories indicate. I feel like I am fairly mechanically-minded, but if you can fix a bike you can probably fix these machines with help from the parts vendors, who I have found to be helpful. If you have a double boiler or HX machine and use it for years, you will eventually have to do some maintenance. Although that said, other than descaling twice a year and repacking the portafilter gasket and screen once a year, I have had to do nothing else. Well, backflush once a week or so.