Mel Gibson flew off the handle during a nuclear argument on January 6, because he had quit smoking that day ... according to Oksana Grigorieva.
Sources familiar with the custody mediation in May tell TMZ ... Oksana said Mel was "enraged" because he quit smoking that day. Oksana claimed Mel tried to stop smoking once a month, to no avail.
Oksana also said Mel had accused her of hanging with fathers from her son's basketball team ... she had been to a game earlier that day.
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