Ever since he started releasing false evidence, er, shlockumentaries. I can't say documentary because there is nothing factual in his little dramas, he and manbearpig have that in common too.
Actually, Moore is master of the half truth....most of his swill contains kernels of facts (Cuba does have a socialist health care system, etc.) which dominate the overall truths of the things he talks and writes about. Cuba does have an excellent health care system...if you are one of the privileged few in Cuba who are elite communist party members. Tourists also can get good health care. Regular, non-commie Cubans get lousy health care to the point of having to bring their own soap, towels, and wash rags to the frequently delapidated hospital.
Another instance, a half truth promulgated by Moore concerned the 2000 election where he and many other libs claimed Miami police intimidated black voters by overpatrolling black poll districts.
As it happened a police car was seen near a poll area in a black district. But it was responding to reports of a crime in process and had nothing to do with intentionally deterring blacks from voting.
And there was his "Bowling For Columbine" scene where he walked into a bank and walked out with a gun trying to make it seem it was easy to get a gun. It later turned out he had to jump through a number of hoops to get that gun. They didn't just hand him a rifle for opening an account.
And there was the "Roger And Me" falsimentary where Moore made it seem like the president of GM refused to talk to him. The fact was the president of GM offered to talk to Moore a number of times.
These are the kinds of half truths Moore fills his books with. He rarely outright lies (although I haven't read all his books), but like many libs he specializes in half truths. Half truths can be worse than outright lies because the molehill of facts obscures the mountain of falsehoods and lies.