One Born Every Minute

One of our characters—usually the one known for buffoonish naivete, such as the Country Mouse, Fish Out of Water, or just someone Too Dumb to Live—befriends a con artist, who promptly sets them up to be ripped off.

One of two things happen:

The victim's friends rally together to set up an even bigger reverse-con to turn the tables on the con-artist.
At the last minute, the con man is touched by the victim's faith in humanity and forswears his evil ways.

The second variant is often used to show the victim as Closer to Earth, his faith in the simple goodness of people trumping the cynicism of his friends.

(Sometimes, they do both, with the counter-con being revealed at the last minute, only to bemuse the con man, who recants anyway.)

Examples of One Born Every Minute include:

Anime and Manga

Played with in Liar Game. Nao's first opponent in the eponymous tournament of conmen is a former teacher of hers who she trusts implicitly. He swindles her out of her stake almost immediately. In order to get it back, she recruits the aid of Akiyama, a notorious conman fresh out of prison for defrauding a corporation whose pyramid scheme ultimately drove his mother to suicide.

Film

In The Great Outdoors, the main characther's brother-in-law cons the main character out of a lot of money, but then has a change of heart.

Live Action Television

The first version was done in Cheers (episode "Pick a Con, Any Con,") Good Times, Saved by the Bell, and Kids Incorporated.
- I think Kids Incorporated did BOTH versions.
The second version was done by Perfect Strangers (with Balki's sinister identical cousin as the con man), and Night Court.
The Golden Girls had an episode ("Charlie's Buddy") in which the con man felt guilty and chose not to take anything from Rose, but he didn't confess to her or give indication that he was giving up his con life.
A twist on the second version was done on The Dick Van Dyke Show, with Buddy's brother Blackie hustling Rob at pool and then tearing up the check in front of Buddy to prove he'd gone legit.
Serena on Gossip Girl in season two.
Hustle, being a show about con artists, has done a number of variations on this, with team members conning one another for fun, conning less honourable crooks, BEING conned by better players of the game... all at once, sometimes...
In the Doctor Who episodes "The Empty Child" and "The Doctor Dances", Rose becomes enamoured by a Time Traveler who saves his life. He also happens to have a very dangerous ship on him, that he'll be willing to give up for a generous price. It turns out that the man is actually a rogue Time Agent whose taken the name Captain Jack Harkness, thinks that Rose and the Doctor are Time Agents themselves, and has positioned the ship to be destroyed once they've paid. The episodes go into type 2, with a twist; the Doctor is not someone you want to cross, and when it turns out the ship is actually dangerous, Jack is horribly guilty and offers a Heroic Sacrifice. (It obviously wasn't completed, since same character is now head of his own show, Torchwood.)

Video Games

Gary in Bully, although he didn't keep up his little charade for very long seeing as he had no reason to.

Western Animation

The Simpsons did this the first version, with the con artist a carny played by the late Jim Varney.
Happens in King of the Hill at least once when Peggy sadly learns that her online Ph.D program is a scam.