My understanding -
If there is an excess of air in the tank, condensation can occur in any outside temp. Witness a car windscreen. The water falls through the fuel and collects at the base of the tank. The engine is fed from near the base. Sod's law clearly states that there will be enough "clean" fuel for you to start up, taxi, complete power checks and commence the take-off run. At the most critical point a dollop of water will be sucked up and deprive your ehgine of power for precisely long enough to spoil your day.
Obviously it's avoidable by carefully draining the tanks pre-flight.
Also, I for one HATE getting to an aircraft to find I need to start, taxi to pumps, fuel etc before I can do anything. As a courtesy to the next user I make sure the tanks are reasonably full. Why not full? Short strip, perf-critical I'm afraid.
Oh well!
