I once boarded last on a CRJ200 (50 seater) COS to DEN. Full plane, sunny day, no wind. GA says they are overloaded, and FA picks a victim to IDB.

It was good to be a 1K that day.

If you fly enough UX, you'll experience many weird things, such as rebalancing pax (moving a bunch to the back). So far, I've never lost my E+ or F seat on EXplus, but I'm sure that will happen.