I've actually seen something worse than this just very recently. When I was flying from DCA on the day of the ZDC nightmare before boarding the GA said we must be in the air by 10pm or we will return to the gate, it was 9:25 and boarding hadn't even started. The flight before the GA reprocessed UGs (many PAX got off the flight with our 4 hour delay) there were 25 pax on the UG list before the delay and only 20 afterwards with 11 empty seats and 0 before. There were also 0 Y seats but after the delay there were 70. The GA has the computer auto do the UGs and we board. At 9:50 we are just about to leave as we only had 10 min to leave and the FAs were rushing as quickly as possible to meet the 10pm curfew when there were 3 empty FC seats still. A PAX sitting behind me had someone he knew is 5B who signaled for him to go up to 5A which was empty. He does and the FA ask another FA why he moved up and she says she has no idea (she's standing in Row 13 exit on a 321 and I'm in row 12). The FA doesn't want to risk having the flight cancelled so she just shrugs it off! Another PAX sitting further back sees the FA doesn't care if he moved up so she just gets up and walks and takes a seat in 3D (3C was still empty and remained open). I checked my DL app to see if anyone had been UGd and it showed 3 empty seats. It's now 9:52 and the door just closed and we took off right at 10pm. Now I'm not complaining as I was just happy to get to ATL that night and the middle seat was open but even when we got in the air the FA never told them to move back and even gave them snack basket etc. Also half way through the flight 2 PAX who were booked on the 11:34pm flight to MCO (we landed at 11:24) came up and just sat in 10E and F for about half the flight. I was rebooked on the 6:55am flight to MCO and just stayed the night in ATL.