These things happen with water and fuel. A ground / instrument school instructor I had came into the class one day and told an interesting story that had happened to him that morning. He had a three hour round trip flight scheduled with another pilot he was checking out. They did a pre flight, took the plane up for 15 minutes, landed and went to a light breakfast before the flight. Told the FBO to top off the plane with fuel. When they came back out the other pilot started pre flighting, and the instructor told her that they just did it like 30 minutes earlier, but she insisted. So he told us, " I am standing there watching her and she is punching the tank, filling the jar and throwing the fuel out onto the tarmac. She does it like five times. So I say hand me the punch cup and I will show you how to do it, you only need about and inch in the bottom of the cup to see if there is water." So he says, "I punch the tank and the fluid is clear with no color. I sniff it. No smell. So I do it like three more times, No fuel. Only water." Heavy rain the night before flowed across the field in the area the underground tanks are filled and one of the caps was loose, the tanker filled up just before topping off their plane, and it is full of water. 7 planes end up grounded with their fuel systems having to be completely decontaminated at the FBO's expense - thankfully none had tried to take off yet. So the moral is, just because you have new fuel does not always mean that it can't accidentally have a little water in it (or a lot as in this true story). The instructor added of course he felt really lucky his student found it, and not when the engine died on takeoff. He said he always punches the fuel now before each flight, regardless, even if they just landed and he is running in to hit the restroom and grab a pepsi before the next flight.