If you saw the media hysteria about a supposed fuel leak as a UAL 767 taxied out, here is what you need to know. This is yet another example of how the media - not only never gets it right - but benefits from falsely alarming anxious fliers.
I flew the 767 for years. It, and also the 747, can spill fuel from the fuel vents when taxiing out when the tanks have been filled for a log flight. The fuel pumpted into the plane is colder than the air temperature. After cool fuel is pumpted in to fill the tanks, the plane sits for an hour or more before the flight. The air temperature raises the temperature of the fuel. On a hot, sunny, day like yesterday, the sun heats the wing where the fuel tanks are. These two factors cause the fuel to expand. The fuel rises up out of the tanks into the fuel vents. Then, the plane taxis out. When turning a corner, fuel swishes around and flows out the vents.
The crew knows this is not a fuel leak. But, if anyone reports it as a possible leak, the plane has to go back to the terminal and be inspected. Pilots generally do not report this because they know what is going on. But I remember an instance some years ago when a 737 pilot reported a 747 as leaking fuel. The 747 captain was furious. He thought the 737 pilot should have recognized this as normal and kept his mouth shut. But once this normal occurance was reported as a possible leak, the captain was forced to go back and check the plane to prove there is no leak.
And then the media gets their bull shit into the mix and reports passengers saved the plane from crashing. http://nypost.com/2017/06/14/united-flight-almost-took-off-with-jet-fuel-gushing-out/
To compare this with something you are familiar with, think of seeing a waiter trying to bring over a martini that has been poured to the very brim.