With a normal fuel load and no malfunction a scenario where you might get a LOW FUEL with an imbalance is leaving the crossfeed valves open.
Over time one side inevitably has more boost pump pressure than the other and an imbalance occurs much faster than with the crossfeeds closed (or so I'm told
). However, you should get a FUEL CONFIG when the imbalance reaches 2200 pounds, normally well before you get a LOW FUEL when the useable fuel drops below 2200 pounds in either main tank.
One time where weird stuff seems to happen with the 767 fuel system is when somebody decides to balance the fuel. United demoed a double engine 'roll back' out of Maui in 2001 and got to check the RAT auto-deploy feature at sunset as they limped into Kona for an overweight landing. Somehow all of the main tank pumps were switched off after fuel balancing on climbout. The DEN ground school lesson on suction feed has now been updated. The star-crossed plane was N666UA but fortunately Captain Luck was in command.
The fuel gauges on the '76 are on some odd ground service bus that I think is used to run 400 Hz vacuum cleaners. I had a jumpseat rider come up to the cockpit and plug his cheap knockoff computer charger into one of the covered AC service outlets without telling me. We were about to go feet wet for a crossing. The fuel quantity gauges went to dashes and half of the landing gear lights started flashing. As I pondered my own overweight landing options, the goofball sitting behind me said 'Look, it quits when I unplug my computer!'
I don't know if it was the fake Dell adapter or maybe the tight plug pushed wires together behind the panel.
There is also a microswitch in the fueling panel door out on the wing that sometimes can cause problems with the fuel indication. There is some trivia question about that on the oral but it's been a long time since I got the rating.