In my case it was pilot/preflight error. Series 5 Kitfox, went out for a late evening short flight, one tank had about 4 gallons and the other maybe 5. I was only planning on about an hour flight all close to my airport so off i went. About 45 minutes into the flight I was flying at about 800 agl and she went silent. this is one of those times where we realize we've got someone watching out for us, i had just made the decision to turn east and fly over a friends house rather than turning west which would have taken me over some bad terrain including a couple small lakes. Fortunately my friends house included a runway which i was able to make a 180 degree turn, slip and land on. The problem was that the only tank that got turned on was the one with only 4 gallons of fuel and my header tank low fuel warning light had failed!!! It was one of those in a hurry, not long till dark, etc etc... mistakes. In my case it all turned out ok but it could have been not so pretty!! The surprising thing was the lack of panic when it happened, i remember my knees starting to knock but not till after i was on the ground. Just remember...PREFLIGHT...