Contamination of the pit with deuterium and tritium, whether accidental or if filled by design, can cause a hydride corrosion, which manifests as pitting corrosion and a growth of a surface coating of pyrophoric plutonium hydride.
The presence of plutonium-240 in the pit material causes increased production of heat and neutrons, impairs fission efficiency and increases the risk of predetonation and fizzle.