What mistake does Gatsby make when telling Daisy about how he acquired his wealth

I believe the only mention of Gatsby's acquisition of wealth takes place when he's talking to Nick on the lawn while waiting for Daisy. His mistake comes when he changes his story about how his wealth was acquired.

“Yes.” His eyes went over it, every arched door and square tower. “It took me just three years to earn the money that bought it.”

“I thought you inherited your money.”

“I did, old sport,” he said automatically, “but I lost most of it in the big panic — the panic of the war.”

I think he hardly knew what he was saying, for when I asked him what business he was in he answered, “That’s my affair,” before he realized that it wasn’t the appropriate reply.

“Oh, I’ve been in several things,” he corrected himself. “I was in the drug business and then I was in the oil business. But I’m not in either one now.” He looked at me with more attention. “Do you mean you’ve been thinking over what I proposed the other night?”