im guilty of forgetting to remove the old gasket...i used one of those k and n filters..the gasket was really thick and i just forgot to remove it when i put the new filter on (im used to it being stuck to the old filter)....i caught myself as i was backing out of the driveway and saw the oil slick...i just cut the engine and look to see what i had dont..

my lesson only cost me a few qts of oil though..

i've seen the double gasket thing happen once...gasket stuck to the block....someone elses **** up...i usually run my finger on the surface to check...i remember hearing the story the following day at work as i left early that day...