Ill pass on a friends experience as had it not been for him it could well have happened to me.
After hiring a PA28 for an early flight to the south of england for a meeting later in the morning, initial run up, takeoff and climb went fine. Weather was clear, but cold. About 10 mins into the cruise the engine began to run very rough so a swift divert and lnding was carried out. Upon inspection and consult of an experienced pilot on the ground it was found to be icing in the fuel. When carrying out the contents check the tanks were on tabs but iced over. This then melted in flight when the sun warmed the wings, settled and entered the engine.
Moral of the story? Always try to move the wings on fuel contents check to check for any ice on the surface of the fuel-particularly on cold mornings!!