Last 1/4 of boarding.
I’m in row 3 and hear the guy walk on and say to the FA in the galley.
“They said at the counter that F is full! There are empty seats.”
(Not really looking up from making the PDB tray) “The plane hasn’t finished boarding sir.”

Not the most egregious case but apparently we should know that once he graces the plane with his presence that they should close the door behind him.