One thing that I've been looking forward to fixing during this year's annual is the slow drip of the left wing sump drain. I wouldn't even categorize it as a leak - maybe on or two drops a day at most. But still, the airport folks aren't going to be overly thrilled about the blue-green drip stain on the hangar floor so I needed to get it fixed.
Intuitively, I thought that removing the old drain and putting in a new one with anything but bone dry tanks would result in quite a bit of fuel doing what any self-respecting fluid would do if given the opportunity to leak through a nice, big, unrestricted hole: GUSHER! I asked the local A&P what he thought, and he stated that if any fuel leaked out, it would only be a little bit.
That sounded good to me, so I proceeded. What I learned was that the definition of a "little bit of gas" in the context of a heated hangar is significantly different than the definition of a "little bit of gas" in the context of a 25 degree Fahrenheit hangar with the gas flowing down your sleeve. Injury to insult: it didn't feel particularly nice when it hit the open cut on my thumb, either!
Cause of the almost-leak: aluminum shavings in the tank had done what they were supposed to do: migrate to the lowest point in the tank. What they didn't do was make it all the way through the drain. There were metal shavings caught between the O-ring and where it seats to the drain, resulting in just enough gap to allow fuel to seep through.