Condensed water in fuel? Anytime there is humid ambient air and a space in the tank above the fuel for it to be drawn in, where water vapour may condense into liquid form. That's why it's preferable to keep the aircraft tanks full when it's on the ground - there's less space for humid air in the tank.
Or someone might pump liquid water in from elsewhere because they didn't do a water check on the bowser.
On the other hand, if you find condensed milk in there, it's certain that some cow has it in for you.
.. is that last retort copyrighted .. or can I use it from time to time ? ... still laughing ..