I've asked a flight attendant to check with a passenger who sat in a middle seat that was supposed to be unoccupied according to the mobile site. I had a look at the offenders BP and it was for a different seat.
I've also sat in the wrong seat and promptly moved when it was made known and I confirmed, or asked if they wouldn't mind taking the exact same seat across the aisle. It's always up to the seat holder to decide IMO.
On some airlines I've had my bulkhead seat changed preemptively to accommodate bassinets, but I've also been asked if it was ok in advance by the check-in agent. YMMV.