Season 4 Episode 2: "Lady's a Charmer"
Lenny Botwin: [complaining to his grandchildren about how Andy was as a child] I asked that prick over there to pedal his little bicycle, which I bought him, up to Del Mar and throw down $100 on a sure thing. Andy Botwin: Father of the year. Lenny Botwin: What the fuck does he do? He pockets it. Goes out and buys himself some Star Trek piece of shit. Andy Botwin: It was Boba Fett. Lenny Botwin: I don't even know what that means!
Turns out that back in the '80s, Andy pocketed $100 that Len gave him to place a bet and opted to buy a Boba Fett figurine instead. That wager would have paid off big for his dad, who has resented Andy ever since. Hence, the $20,000 that Andy "stole" from him. (Source: Entertainment Weekly)