Oksana Grigorieva's former bodyguard -- the man who knows her playbook inside and out -- was subpoenaed today in her custody war with Mel Gibson ... and he got served while he was blabbing about the case.
Sources connected to the case tell us Kristian Herzog (left) was served on Wednesday as he left one of several interviews he did about Mel and Oksana. One of his stops was with Mike Von Fremd (center) of ABC News.
As for why Mel's attorneys want to get Herzog in court -- we're told he was in virtually every lawyer's meeting that Oksana had last year and knows all her secrets.
Most importantly ... he is not bound by attorney-client privilege -- which makes him the perfect fly on the wall.