Season 4 Episode 2: "Lady's a Charmer"
Andy: "It was Boba Fett!"
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)