Sheldon and Penny
Sheldon and PennyTwitter/The Big Bang Theory

Season 10 of The Big Bang Theory is inching towards its finale and speculations are rife that both Leonard and Penny and Sheldon and Amy's relationships will be in trouble, thanks to people from their past.

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The penultimate episode of the season, which will air on Thursday, will see Penny's former boyfriend Zack entering the picture and this makes Leonard uncomfortable. Leonard and Penny have been bickering a lot from the start of the season and it wouldn't be surprising if they decide to take a break from each other.

The Big Bang Theory executive producer Steve Molaro hinted Zack's return could cause a friction in Penny and Leonard's marriage. While speaking with TV Line, the showrunner said something Raj said to the couple in the past will be a cause for concern for Leonard.

"Leonard is definitely uncomfortable with the thought of Penny going to work for her old boyfriend. I don't know if he thinks she'll be unfaithful, but according to Koothrappali, she's gonna leave Leonard eventually – why not get it over with," he said.

As for Sheldon and Amy's romance, the finale will see Dr Ramona Nowitzki, a former Sheldon admirer, turning up while Amy is away, and the gang isn't happy about this. "Sheldon is smart, but he may not be smart enough to realise he's being aggressively pursued by a brilliant, former Olympic swimmer," noted Molaro.

Will the season end with these two central relationships in jeopardy? Will Sheldon and Leonard move back together?

Season 10 episode 22 titled The Cognition Regeneration reads: "Penny's job offer from a former boyfriend doesn't sit well with Leonard."

The Big Bang Theory airs Thursdays at 10 pm ET on CBS.