I was just thinking about this topic a few weeks ago when I flew home from Frankfurt. We were comfortably settled into our seats 21H/J ^. A woman sat down a couple rows up in G, stowed her stuff, put the pillow behind her head and wrapped herself up in the blanket. About 20 minutes later (it took a long time to board and E+ filled up on the late side), a man came along with a bp for that seat. It was very obvious from her reaction that she knew she wasn't in the correct seat. So, she very slowly moved to the middle (E or F), leaving the now-used blanket and pillow lying in G. Same flight, one of the flight attendants had to shoo several people out of the crew rest seats during boarding--despite the signs taped to the seats and o/h bins.

Also, my husband recently bought E+ at check-in and was lucky enough to get 6C on the Airbus. But he had the last boarding group, and found someone already in his seat. Turns out, that person had a different seat assignment, but someone else (who never had 6C in the first place) had asked him to switch it for 6C. Husband's reply when asked where the person now in 6C was supposed to go: "Sorry, not my problem."