In just about every episode of The Flintstones that mentions Wilma’s maiden name, it’s Slaghoople. When Fred's mother-in-law comes to live with the family in the March 1962 episode "Trouble-in-Law," it’s Pearl Slaghoople who shows up at the Flintstones' humble home. In the February 1963 episode "Mother-in-Law’s Visit," Pearl Slaghoople shows up again, just before little Pebbles is born. And in the hilarious fourth season episode "Bachelor Daze," which finds Fred, Wilma, Barney and Betty in a flashback recalling how they first met, Fred recalls the very first time he met his future mother-in-law, Mrs. Slaghoople.

On the issue of Wilma's maiden name, Hanna-Barbera canon upholds that it's certainly Slaghoople.

Except that, two months before Pearl Slaghoople showed up, Wilma's maiden name had been "Pebble." In the January 1962 episode "The Entertainer," Wilma runs into an old high school pal, once "the most popular girl in school." 

"Greta! Greta Gravel!" Wilma exclaims to the blonde. Greta replies, "Wilma! Wilma Pebble!" To which Wilma says, "Not anymore! I'm married now."

Again, in a stray episode between "Trouble-in-Law" and "Mother-in-Law’s Visit," The Flintstones again introduced confusion by giving us "Dial S for Suspicion."

In that December 1962 episode, Wilma's maiden name isn’t Slaghoople, but Pebble.

In "Dial S for Suspicion," The Flintstones goes a little noir when Fred answers a want ad for a better-paying job at a ritzy resort. When his application is accepted, Wilma's first request isn’t for fancy prehistoric items like "meat, diamonds or anything like that," as Fred suggests, but for Fred to get life insurance.

Upon discovering Wilma's reading a crime novel about a woman who kills her husband to claim insurance money, Fred starts to worry that Wilma might be trying to off him for a $40,000 payout.

Barney tries to calm these fears, telling Fred, "Don’t get any ideas about Wilma. She’s true blue!"

But when the broad blade of Wilma’s meat cleaver flies off the handle and nearly takes off his head, Fred's suspicions ramp up and comedy ensues.

About 15 minutes into the episode, a subplot finds Wilma running into an old boyfriend at a parade with Betty. When the old boyfriend greets her quite enthusiastically, he doesn’t say, "Wilma? Wilma Pebble!"

And he doesn't just say this foreign name once.

As the conversation goes on, he repeats this full name, "Wilma Pebble, you look beautiful as ever!" Blushing, Wilma says, "Oh, Rodney, but I’m not Wilma Pebble anymore." She tells him of her marriage to Fred, and then the episode weaves back into the main plot regarding Fred's suspicions.

This is the only episode where Wilma’s called Wilma Pebble. It originally aired four months before The Flintstones would introduce Fred and Wilma's daughter, Pebbles.

Some fans have tried to fix these inconsistencies by speculating that Pebble is Wilma's middle name, making her the former "Wilma Pebble Slaghoople." But, Flintstones writer Earl Kress once simply explained, "It's just as simple as Hanna-Barbera not caring about the continuity."

But, perhaps the name "Wilma Pebble" was so cute that, when thinking of a name for Wilma’s daughter, writers remembered this tiny goof and adopted Wilma’s discarded maiden name to give to the new baby?