I've been dopey and sat in 2D (my regular seat) on one flight when I had to pick 2C because 2D and other aisle seats in CE except the last row were occupied. Of course I forgot about this exceptional situation and sat in 2D. I was highly embarrassed when the rightful occupant turned up.

However, I've never accidentally sat in a wrong cabin, ever, or accidentally taken a seat in ET when I was ticketed for CE. When I am flying ET, it usually bugs me for some time before the flight that I cannot possibly forget that I'm sitting in ET