I've seen it a few times. What often happens is that their seat-preassign engine assignes Y pax the seats in the last rows of Y. And then they forget to change the seating plan when M is overbooked, and suddenly Y gets filled with random op-ups.

Naturally, op-ups from M to Y isn't very costly for them, so I guess they rather do that than to reseat the entire Y cabin. I am sure that revenue management is more picky about the C/Y divider.

I've also seen the "we don't care to change the headrests" phenomenon. Once I was interrogated by some grumpy FA because I had a seat in 7A or something, which had a Y headrest but I was in P-class. After disappearing with my boarding pass for 5 minutes she handed it back and said nothing more. Naturally there was no food given to me.