I know this is caused by leaving the steam valve closed when you turn off the machine. The drop in temperature causes a vacuum of pressure and when you open the valve, especially if you open it with a pitcher of milk submerged it will either suck in the milk residue left in the steam wand or a full shot of milk. Then the milk just does its thing. This is what an espresso technician told me of an Oscar that was sitting on his shelf. I would like more info as to the process of descaling the machine, though, as well. I have been scared into not wanting to do it myself, and have been convinced all you need to do is backflush unless something drastic occurs.