279 episodes is a lot of work and writing. For those behind the scenes on The Big Bang Theory, it proved to be a challenge at times. In the following, we're going to take a closer look at regrettable storylines, and episodes that fell short of expectations for the fans. One episode in particular didn't sit well with showrunner Steve Molaro, so much so that he spent the entire hiatus trying to undo a storyline that wrapped up season 4.

As for the show's worst rated episode, IMDb named a season 10 episode among the biggest duds of the series. We'll take a closer look at the poor rating, and what went on during the main plot.

It featured a return of a guest star that appeared numerous times as an ex of Penny. However, it seems like his return wasn't enough to save the show from a negative rating. As Kaley Cuoco herself revealed, it was nearly impossible for the show to produce episodes without a dud sporadically.

The Big Bang Theory Had Several Forgettable Episodes Including The Season 4 Finale

Ending a season with a strong storyline is no easy task, and showrunner Steve Molaro felt immediate regret during season 4's last episode.

Chuck Lorre wanted to end the season with a 'bang', literally... but Molaro was not pleased with the end result, and neither were lots of fans. The final moments see Penny and Raj coming out of bed together. Showrunner Steve Molaro instantly plotted on undoing the story once the season wrapped up.

"I really did not like that season finale ending," Molaro admits.

"I spent the entire hiatus being upset about it… I went upstairs to [Lorre's] office right when we got back to work [on Season 5] and said something like, 'This is really bothering me. I think I have a way that we can… get out of it and preserve the characters as they are.

"

Molaro would go on to reveal alongside TV Line that Lorre wasn't exactly thrilled to undo things, but came up with the perfect resolution and one that settled lots of fans down.

Zack's Return Wasn't Enough To Save The Lowest Rated Episode In Season 10, The Cognition Regeneration

The lowest rated episode took place in late-April of 2017, entitled The Cognition Regeneration. IMDb rated the season 10 episode with a rating of 6.7 stars, the lowest out of all The Big Bang Theory episodes. The main plotline features the return of Penny's ex Zack, who was generally well-received by fans.

In the return, Zack offers Penny a job, which doesn't sit well with Leonard.

Although the episode didn't receive a strong rating, the man behind the character of Zack, Brian Thomas Smith had a blast on the series, which was only supposed to be for one episode. He became a recurring guest-star, and had praise for his time alongside the entire cast.

"Working with the cast of the BBT was an amazing experience. Everyone on set was such a professional. From the table read to the live studio audience taping, everyone was always on their A game."

Smith continues, "I was initially supposed to be a guest star on just one episode, so it really came as a surprise when the cast readily welcomed me.

The cast didn't have to be so friendly, especially since I was supposed to be gone in a week."

A positive experience for the actor, but it seems like the showrunners behind the scenes struggled with Zack-related storylines in the past.

It Wasn't The Only Time Big Bang Theory Creators Debated A Storyline For Zack On The Series

Showrunner Steve Holland revealed that another major storyline with Zack almost took place, and was heavily debated in the writers' room. The showrunner states that since Penny and Leonard weren't having kids, or at least at the time that wasn't the plan, they tinkered with the possibility of Leonard giving Zack and his new wife his sperm.

Holland revealed, "We talked about going down [that] route where Leonard was the sperm donor… but the conclusion we came to was that it wouldn't feel OK. People had different opinions about it in the writers room," but the general consensus was "that it wasn't going to be satisfying."

As for Kaley Cuoco, well as an observer alone, she was completely against the possibility.

"Listen, there's a lot of storylines you need if you're going to run 12 years," she says. "You're going to hit a doozy once in a while!"

Ultimately, fans were in for quite the swerve towards the end of the final season, as Penny announces that she's pregnant with Leonard's child, despite the fact that it seemed as though children were not in the couple's future.