Originally Posted by ual744777sta
Middle seat blocking is only guarenteed until the flight is handed over to airport control. After that any CSR can place someone next to you. They usually unblock the seats toward the front of Y and this happens regardless of status.
Middle seat blocking is only a courtesy block. At anytime, the middle seat can be filled.
For example, I just purchased a ticket on a flight where every window and aisle E+ seat was assigned. The seat map online showed that the middles were not available due to courtesy blocking. I was not going to sit in one of the 5 empty back of the plane E- seats. I called up reservations and asked if a row 6 middle on the A319 was available and she put me on hold and had 6B unblocked and assigned to me.
Sorry to the two assigned in 6A and 6C, but 6B/6E are the best middle seats to get stuck in in E+ if nothing opens up.