I've had a flight gate changed very late (maybe T-40?) from B-something to A10 (the plane stayed at the B gate, but pax were bussed there). Absolute chaos for pax to get back from B to A (most don't know about the tunnel) and probably only about 20% were at A10 by T-20. I don't exactly remember when boarding closed but I'm sure a few missed the flight either by failing to find their way back quick enough or by trying to use the transit and ending up at immigration.