Kevin O'Leary Recalls His Worst-Ever 'Shark Tank' Investment That Lost Him Half a Million Dollars

Kevin O'Leary wasn't feeling so wonderful after he made an investment on Shark Tank that ultimately lost him half a million dollars. As the hit ABC series enters its 14th season, Mr. Wonderful looked back on the bust as well as the lesson he took from the snafu — always listen to your gut.

"What I've learned after doing this a long time [is] no matter how you feel, and how optimistic you are, in the moment of making an investment, pulling the trigger, you have no idea of the outcome," O'Leary told CNBC Make It Thursday. "You have zero probability... Anybody that says they do is full of s—."

O'Leary declined to call out the Shark Tank idea he invested in that didn't work out, citing ongoing litigation, but he did reveal that the investment was made into a telecommunications startup and he lost $500,000 by supporting the deal. At first, he put $250,000 into the idea. Four months later, the startup founder called O'Leary asking for another $250,000 because he "burned through" O'Leary's initial investment.

This immediately sent red flags flying in O'Leary's gut, but the startup founder insisted he would correct his mistakes and fix his ideas. "In my stomach, I didn't feel right about it," O'Leary told CNBC. "My gut said 'No.' But because I knew the guy and I liked him, and he was a friend, and yada, yada, yada ... I gave him another [$250,000]."

The founder did not keep to his promises, though. He ran through O'Leary's second investment in just two months and there was nothing to show for it. "He went to zero 60 days later. So I lost half a million dollars," O'Leary said.

The big lesson from this situation is to "listen to your gut," O'Leary said, "because that is your experience talking." He also noted that many stories involving bad investments include a founder insisting they can change their ways but refuse to. "They can't get out of their own way. They won't listen to anybody else," O'Leary said. These entrepreneurs are "stupid," letting their stubbornness keep them from listening to anyone who could genuinely help.

O'Leary admitted that most investments aren't going to be big hits. However, the big hits should cover the losses from the failures. The worst Shark Tank investment still stings though. "I ignored it, and it cost me half a million bucks," O'Leary told CNBC. "That was really stupid. I'm pissed. And I don't forget that."

Shark Tank Season 14 debuts on Friday, Sept. 23 at 8 p.m. ET on ABC. Gwyneth Paltrow and DoorDash CEO Tony Xu will make their guest shark debuts this season. Mark Cuban, Barbara Corcoran, Lori Greiner, Robert Herjavec, and Draymond John will join O'Leary as the main shark panel.