Golf's worst-kept secret is that Phil Mickelson likes to, um, wager on his own game.
One guy on Reddit has a personal example from a pro-am he played with Lefty in 2002.
The guy was playing with Mike Weir (that really was a long time ago..) and said Mickelson ran up late to join their group.
He tried making small talk which Mickelson wasn't having anything to do with before saying "hey, let's play for some cash today."
What followed was fantastic:
"To which he turns around in his seat, looks me square in the eyes, and says, 'sure, how much were you thinking?' At this point I'm about to piss myself, thinking I've just agreed to play a $100,000 round of golf or something, but once we get out of the shuttle he tells me we will play for $20, if that's okay, and he would factor in my handicap (he gave me 8 strokes). He was really nice about it and told me to find him in the driving range afterward and to bring my scorecard."
He went on to say that Phil paid him $20 from a balled up wad of about $5,000. So that's the best pro-am story I've ever heard.