A very experienced instructor friend of mine once told me a story of how he reckoned he was 2 mins from double engine failure in a Seneca (with one tank still full!) . During the taxy checks the cross feeds are checked. In this case his student didn't return one of the cross feeds to ON - and his mistake, he didn't check. The result was that both engines were feeding from the same tank. The "problem" is that the left engine returns fuel to the left tank, and right to the right tank - the result with full tanks is that one tank is being drained at 30 gals per hour, and the other full tank is being fed with return fuel, which gets dumped overboard if the tank is full.
Of course when they noticed this, they crossfed from the other tank and everything was ok.