Oil fired forced-air installation had the flue pipe jammed on the breech so far that the flue pipe extended into the fan plenum. Breech draft was -.04, presumably the fan plenum pressure would drop well below -.04 when the fan starts. This would cause flue gas to be sucked into the return air, and there was a plume of soot on the outside of the breech pipe that passes through the fan plenum.
I'd guess that a proper installation keeps that external flue pipe well outside of the fan plenum. About how far from the furnace wall do you typically keep the end of the external flue pipe?