Interesting incident at Leuchars on 1B07 today. Was already 7 down on arrival.
Anyone who knows the layout will know HST stop board is at the far end of the platform, meaning the main pax stairs onto the platforms at the north end.
Guard has walked the platform, confirmed doors closed and was away to step back into open guards door after whistling the service good to go (or had just done so, can't remember) when a late passenger attempted to step into the door to board.
The guard correctly invited him to step down and then explained via the drop light presumably that this was not a customer door.
Customer certainly wasn't impressed, and unless I've missed it can't see any mention in the Classic intro video that Guards door = no admittance.
Daresay travelers, including myself, have been used to diving in unit doors at last minute.