The Hidden Big Bang Theory Detail Most Fans Never Notice

For nearly a decade, Apartment 4B served as the living quarters of Penny (Kaley Cuoco) on "The Big Bang Theory." It's a place where she can decompress after a long day at The Cheesecake Factory, sip wine with her girlfriends, and escape the daily tirade of insults from Sheldon Cooper (Jim Parsons). In many ways, Penny's apartment is symbolic of her personality: a little imperfect, but also vibrant and welcoming.

A perfect representation of this is Penny's refrigerator, which is adorned with haphazardly placed photographs of her smiling with various friends and family members... or so it appears. Most "Big Bang Theory" fans never noticed that several of the snapshots aren't staged photos of Penny, but rather Cuoco on set and backstage with various cast and crew members. It's a perfectly subtle memento of the group's unforgettable times bringing "Big Bang Theory" to life.

Over the years, the photos change to include Melissa Rauch (whom Cuoco gets along with in real life), Mayim Bialik, and, of course, the late Carol Ann Susi, who unforgettably plays Howard Wolowitz's (Simon Helberg) unseen — but very much heard — mother. One eagle-eyed fan on Reddit noticed that one particular shot of Cuoco and Johnny Galecki (who plays Penny's eventual husband Leonard Hofstadter) was taken while filming Leonard's dream sequence in Season 5, Episode 13 ("The Recombination Hypothesis"), where he imagines rescuing Penny from the apartment building's broken elevator.

Capturing precious moments through photographs is an endearing character trait of Penny — as well as Kaley Cuoco. Throughout the impressive 12-season tenure of "The Big Bang Theory," Cuoco made it her mission to chronicle as much behind-the-scenes hilarity as possible. By the time the show wrapped up, she had snapped thousands of Polaroids as a special way of capturing memories on set.

From candids in the makeup chair and costumed gatherings between takes to smiling group shots that show the cast's camaraderie, Cuoco's collection is a gold mine for fans of the show. Though she kept many of these shots private for some time, only sharing a select few when Instagram came into existence in 2010, they're now available for all to see — and you don't have to squint at a televised refrigerator to get a good look.

Many of Cuoco's Polaroids can be viewed in "The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series," which features 16 pages of photos from her personal collection. In an interview with Glamour about the book's release, Cuoco said, "I'm so excited for fans to finally see my behind-the-scenes Polaroids for the first time. These photos are very special and super personal."