This door, worked in an elaborate frieze displaying some glorious scene out of history or legend, is in fact a carefully designed trap. Hollow, and balanced carefully on a horizontal pivot while giving the appearance of opening normally, the lower portion of the door contains a liquid; meanwhile, the upper portion contains a powder, kept separate by an easily-dislodged plate. The trap is sprung when the door is disturbed; pivoting on the horizontal axis, the powder and the liquid mix freely, reacting to create a deadly gas that erupts out the myriad hidden holes amid the elaborate frieze, giving the door the appearance of belching the deadly fumes out from countless nostrils, mouths, and eyes.
Generally, this door is used to guard sealed areas which none are meant to enter, or doors which the real owner of the area can bypass by some means, be it a concealed door nearby or magic to skip past it entirely. Particularly paranoid and antisocial individuals might even employ them in the living spaces, although they will tend to reside on a higher floor, safe from the dense fumes.
Additional Ideas (1)
When the adventurers reach this point of the maze/dungeon/house, they find that someone has already half opened the door. It is moved from the vertical position and hangs at an odd angle, leaving narrow gaps at the bottom and top. The only problem with crawling under it is the gas - it has been expelled from the door and now covers the floor in a thick layer of brightly coloured fumes (maybe even concealing the lower gap). It is a matter of wits and acrobatics to pass the trap now, without dropping into the poisonous cloud. A nasty surprise may be the body of the one that opened the door, hidden in the gasses that swirl on the ground (something to trip over).