There was a software issue (I think) that assumed an incorrect Velocity of Sound (VOS). This caused incorrect fuel quantities to be displayed.
Another problem was during sustained cold weather operations. Water freezing at the bottom of the fuel tanks would over time cause a build up of sheet ice within the tank-because the ice doesn't melt in winter on the ground. The distance between the fuel surface and datum point IE the tank base would appear incorrect to the sonic 'ping' causing odd fuel imbalance status messages in flight. The story goes that Continental Airlines entered a fuel tank to try and get to the bottom the problem and found a sheet of ice several inches thick.
Finding Ice isn't new. The way it causes problems on the cripple is.
Hope this helps.
