Over ten years ago now, I checked the oil level pre-flight, but failed to secure the dipstick properly on my R-R C-90 powered light aircraft. The flight was only 15 mins. There were no cockpit indications - noise or oil pressure - but the amount of oil lost, only a few cc's, went a long way in staining the fuselage side. There was no way I could miss it on the after flight, or the next walk-roound!
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