Just a few oil changes ago I started pouring oil back into the crankcase without putting the plug back in. I'd poured less than 1/2 a quart when I thought about what I'd done. I was outside on a gravel drive so no mess to clean up. These are reasons we SHOULD ALWAYS check that the plug has been put back in before pouring and ALWAYS check for leaks when we're finished. Knock on wood. I've never had a double gasket. I remember it happening to my dad once, ever since I take a rag and wipe the filter sealing surface and check that there's not a gasket there.