Well, the lavatories weren't properly constructed when originally built and the storage overflow were built in rectangular shape while the the auxiliary fuel tanks were constructed in pyramidal rows of six, each auxiliary pyramid placed at 90 degree angle formation to each other for maximum dynamic flow. But the rectangular shaped lavatory storage overflow ended up at an obtuse angle configuration that corroded the overflow capacitors allowing the lavatory overflow to leak into the auxiliary fuel tanks, which was the same conditions that resulted in the blowout of the Kursk double-hulled submarine. I'm sure you realize the fuel toxicity generated put the aircraft in grave danger. Below are now declassified documents that explain the entire procedure.
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The lavatory compartments were rebuilt and completed today as of April 20, 2014 which the Russian Air Force chose to commemorate by flying the plane out 50 nautical miles and using the new upgraded lavatories in celebration.