Today was the day for a routine oil change on my X-Town, 400 miles overdue, but better late than never. All went very smoothly until I drained the old oil into the container I use for that purpose and found, lurking in the bottom, the crush washer!!
Yes I had, by this point, replaced the sump nut and filled up the oil, all one litre of it. Blast! I'm figuring the crush washer is there for a purpose so there's nothing for it but to drain all the oil again so I can replace it. Now a stroke of luck. By chance I'd left the dipstick screwed home and as a result the engine oil only came out relatively slowly. It was messy, but I was able to remove the sump nut, replace the crush washer and screw it home again with only about 10-15 centilitres dumping into the oil pan. Yay!
So to rephrase the old carpentry axiom about measure twice, cut once, I suggest a mental tick list of ALL the steps in an oil change to avoid the sort of schoolboy error I just made.