Well, at risk once again of receiving the ridicule I deserve I will relate the following true tale:
A few weeks ago I began noticing unpleasant tastes in my espresso shots and they were even noticeable in milk drinks. I had changed my roasting parameters recently and began to suspect that this was the cause. I went back to my old protocol, but the tastes were still there. I tried a few different beans, but they all seemed to give me that unpleasantness, aside from monsooned Malabar, that is already of a different nature. I began to think it was my taste, but then Karin said, "This coffee is, uh, kind of strange." I slunk down in my chair and grumbled that I hadn't noticed.
After a few days of this I made an AeroPress cup and it was fine. This surprised me until I suddenly remembered that I had not done a proper cleaning of my espresso machine for a few weeks.
I actually was a bit surprised that not using machine cleaner and flushing, boiling the portafilters and baskets could make that much of a difference, but it does. What can I say?