The smell of oil...
So my daily driver has a little oil leak tucked somewhere up above the oil filter, probably from a gasket. Every now and then when the car gets really hot I can smell the oil burning off the bottom of the engine block. I panic until I remind myself of the non-serious issue.
When I'm out riding my bicycle and I smell oil, I get the same sense of panic and my eyes wander around the drive train, as if I expect there to be oil spurting out from the bottom bracket.
Le sigh.
-Gregory