My favorite case was where someone was accused of arson because the wood floor was found to contain traces of gasoline, even though none was found on the carpet or padding on top. Turns out that further testing by the defense showed lead also. Which means that the gasoline came from a cheap contractor several decades ago that used (at the time leaded) gasoline as a lacquer thinner for the hardwood floor. And the burn patterns on the floor which indicated an accelerant, were due to wear patterns on the carpet above which allowed more heat to penetrate.