Tim Donaghy. He casually bet on NBA games that he wasn't involved in with his golf buddies, who unbeknownst to him were passing his tips/bets on to organized crime. Donaghy decided to stop betting because it hit him that he was abusing his position and would be in deep, deep s*** if anyone found out, and the mob (obviously) wasn't happy. They tracked him down early in the 06-07 season and threatened him back not only into picking games (directly for them this time), but also to start betting on games that he officiated.
He took note of league directives that went out to refs through e-mail (i.e. they would send a bunch of replays of Allen Iverson getting away with palming violations, the obvious message being that the league wanted to refs to crack down on that), and he knew the officiating patterns and biases of his fellow refs well enough to hit on like 75% of his picks. He knew that Dick Bavetta officiated to keep games close, so he bet the underdog in Bavetta's games on the assumption that the favorite would fail to cover the point spread due to Bavetta manipulating the game. He knew that Joey Crawford hated several league owners and would try to stick it to their team whenever possible. Allen Iverson once publicly mouthed off about Steve Javie and didn't really get punished by the league for it, so the refs not-so-secretly decided to inflict mob justice and screwed Iverson on calls for the next couple of weeks, which Donaghy used to make more picks and win more money by betting against Denver for the next couple of weeks. Interestingly, a bunch of refs were pals with Isiah Thomas and would hit him up for signed merchandise and other goodies, with an unspoken understanding that they would help out the Knicks on the court, which makes the Knicks' failures in the mid 2000s all the more hilarious - they had several officials in their back pocket and STILL could only win 23 games a year. Maybe a certain ref blew an important call that cost Orlando a game and felt guilty or was doubting himself over it. If that ref mentioned to Donaghy or Donaghy otherwise found out that the ref would be working another game for Orlando within the next week or two, Donaghy would pick Orlando to win that game.
Sometimes the rationale for his picks was less sinister - maybe Kobe Bryant or Allen Iverson somebody was the favorite player of some ref's son or nephew, so the ref would favor that player, subconsciously or not. Maybe a coach owned a restaurant and Donaghy found out that the restaurant comped the ref's meal, or gave them a standing 'free dinner' card for whenever they were in town, and of course they would return the favor and help his team out on the floor.
I have his book. Pretty interesting stuff.