Some people just can't seem to make it work. They go through divorce after divorce and never seem to learn their lesson. These characters are typically middle-aged, otherwise successful men, but none of these are requirements. The only requirement is that the character has been through at least two divorces.
There are numerous reasons for a character's marriages to repeatedly fail. Sometimes a needy best friend drives a wedge. Sometimes the character is still too devoted to a lost loved one. Usually, it's because they're simply Married to the Job.
This trope is often part of a character's Dark and Troubled Past, but it can also be played up for comedy. In both comedies and dramas, the character is often part of a Dysfunction Junction.
Bear in mind that an end of a marriage doesn't necessarily constitute a divorce. If the character has been married twice, but one of the marriages ended in death, it doesn't count.
In the film version of Jurassic Park, Ian Malcolm comments at one point that he's always looking for the next ex-Mrs. Malcolm. It's implied that all of his marriages ended in divorce.
The movie The Marrying Man centered on a couple who married each other and then divorced multiple times over the span of decades. By the end of the movie they are about to get married for the fourth time.
In The Heartbreak Kid, Ben Stiller character Eddie rushes into marriage with Lila. During the honeymoon, he realizes how mismatched they are and meets another woman Miranda. After many mishaps, both women leave him. Years later, he is settled in Mexico, and Miranda visits him. In the final scene he makes plans to go on a date with her and after she leaves, it is revealed that he is married to a third woman, and possibly is going to divorce her to be with the One.
Ma Flodder from the Flodder movies has been married 5 times, but is currently single again.
In The Mark of the Lion trilogy, Julia Valerian goes through three husbands, one fiancé, and multiple lovers. Two of the husbands die, but relations with the fiancé and one of the husbands are broken off by divorce.
1066 and All That's genealogical table of English kings shows John of Gaunt as having been married to Lady Godiva, Margaret of Angoulème, Joan of Arc, The Infanta, The Maria Theresa, Ann of Geierstein, etc. and etc. etc. The text also states that Henry VIII had VIII wives. There was also the Scottish queen Mary, who had so many husbands that she was known as Mary Queen of Hearts.
Helene Roffe-Martel in Sidney Sheldon'sBloodline has several ex-husbands. All of whom she married to become wealthier than she already was. Her current (at the book's time) husband is a henpecked one she married because he was a nothing she could mould into whatever she wanted.
On Castle, the titular character has two ex-wives.
On Law & Order, Jack McCoy has been divorced twice. Lennie Briscoe also has multiple ex-wives, although the exact number is unknown.
Guinan on Star Trek: The Next Generation has been married 23 times. It's not clear if all these marriages ended in divorce, or if she outlived some or most of her husbands.
This trope is standard practice for Trill symbiotes, as their marriages are considered to end automatically if they transfer to a new body. Dax on Deep Space Nine had been married so many times in various former host-bodies that she insisted on a Klingon wedding to Worf, simply because it was a kind she'd never had before.
Ross on Friends had three ex-wives by the end of the series.
Ross: I'm gonna go out on a limb here, but No Divorces in '99!
On Community, Pierce has been divorced seven times.
Al Calavicci of Quantum Leap had five wives, three of which are confirmed to have left him in a divorce (or divorce-like circumstances), and one is implied to. Only the first one really "worked": She only left him because she waited long enough after he was declared dead.
On My Name Is Earl, Earl has been married three times. The first time was an Accidental Marriage to Joy, until she left him (while he was hospitalized, no less!) for Darnell. The second time was to a friend's elderly mother, until she wanted sex and he didn't want to give it to her. The third time was to a friend's ex-girlfriend who wanted to follow in his footsteps...then morphed into a Yandere when he was more focused on the list than on her, before she finally left him to join an Amish-like settlement, doing him a final favor by leaving him all the money she got in her court settlement. Earl states that despite all their problems, Joy was his favorite wife.
Babylon 5: Eventually revealed in the 5th season to be the case for Sheridan (three wives, including current and final wife Delenn). Lampshaded by Garibaldi, wondering just how many wives Sheridan has.
On Scrubs, JD mentions that after his parents got divorced, his mother "started marrying everyone who rang the doorbell."
Mythology and Religion
In The Bible, the unnamed Samaritan woman who chats with Jesus at the well has been married and divorced five times, making her a social pariah. (Which is why she went to the well by herself in the middle of the day, rather than at dawn or at dusk like most women would.) She was apparently living with a boyfriend (who likely already had a wife, making her The Mistress).
Millionairess Claire Zachanassian in Friedrich Dürrenmatt's play The Visit. Over the course the play, she divorces her seventh husband, marries and divorces the eighth, and prepares to marry the ninth.
One of Marge's sisters on The Simpsons: Selma Bouvier-Terwilliger-Hutz-McClure-Anderson-Simpson-D'Amico. She refused to have a Citizenship Marriage with Apu Nahasapeemapetilon just because his name is too long.
My name's already Selma Bouvier-Terwilliger-Hutz-McClure. God knows it's long enough without Nahassapeema-whatever.
In fact, the only marriage she had that didn't(technically) end in divorce was to Disco Stu, who got an annulment from the Catholic church.
Also, Lurleen Lumpkin (a country singer that tried to seduce Homer) has been married 5 or 6 times, all to men that look and act just like Homer.
In Family Guy, Lois' sister, Carol has been divorced nine times. She's currently married to Mayor Adam West.
On Hey Arnold!, Arnold has been left some money by a lounge singer that he had helped to rediscover his talents in an earlier episode. At the will-reading, are several different blond women that he had been married to at one point or another.
On Daria, Jane's oldest brother Wind is on his third or fourth wife by the time he first shows up. In this case it seems to be because he's a neurotic mess who jumps into relationships with less-than-stellar women (he insists Wife #3 "isn't like the other ones").
Relative: And [how's] your brother Wind?
Jane: He's thinking about getting remarried if he can just figure out whether his divorces were legal.
Elizabeth Taylor was married eight times including twice to Richard Burton. Her third marriage ended when her husband died but all the other ones ended in divorce.
Burton himself was married five times, including his two marriages to Taylor. Only his last one didn't end in divorce.
Larry King was married eight times, to seven different women. All but the last marriage (his longest one yet) have ended in divorce.
Zsa Zsa Gabor is currently living with her ninth husband. All of her previous marriages ended in divorce, except one that was annulled.
Her older sister Magda was married six times, and her younger sister Eva was married five times. Their mother was married thrice. Guess it's in the blood?
Mickey Rooney has been married eight times and divorced seven times. He has been living with his eighth wife for 33 years, which is more than the length of all of his previous marriages combined.
Henry VIII of England is probably the...er, king of this trope. He created an entire church (with him as the boss) in order to get his marriage annulled. He married six times in total; two of his marriages were annulled, three ended in the wife's death (twice with a bit of help from Henry), and the last ended with Henry's death.
Baptist minister Glynn Wolfe, married 29 times, altough not all his marriages ended in divorce. His last marriage, solely a publicity stunt, was to Linda Lou Essex, who was married 22 times before.
Norman Mailer was married six times (though his marriage to Carol Stevens, which lasted for one day, was only to legitimise the child they had together).
Musician Ben Folds has been married and divorced 4 times, and currently has a girlfriend.