On a short cross-country in a C-150 I pulled the cabin heat on and loaded
the cockpit with gasoline fumes. I immediately closed the heater vent but
the fumes didn't go away. Leaving a window opened at least brought in some
fresh air. Made a precautionary landing about half-way to my destination.
When I did the GUMP list I felt the carpet soaked with fuel. Turns out the
fuel shut-off valve packing was dribbling gasoline into the cabin. When I
opened the cabin heat vent, the air movement stirred up the fumes that were
collecting below my knees.

By the way, just to stir up a previous thread -- when I declared my
intentions to make a precautionary landing (without declaring an emergency)
the tower rolled the equipment. The only flak I caught was from the fire
chief for taxiing the airplane off the runway to the apron. In hindsight, I
probably should have stopped and ran -- like he said.