The Big Bang Theory Prank That Landed Kaley Cuoco In The Hospital

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Sometimes, real-life accidents prevent actors from appearing in projects — for example, on "The Big Bang Theory," Kaley Cuoco missed two episodes as Penny because of a real (and very serious) injury she incurred while horseback riding. According to Jessica Radloff's book "The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series," which hit shelves in 2022, Cuoco and her co-star Johnny Galecki, who plays Penny's on-again, off-again love interest Leonard Hofstadter on the series, planned a prank where they would stage a fake punch. Unfortunately for literally everybody, it went horribly wrong.

The prank in question takes place in the season 6 episode "The Tangible Affection Proof," where a seriously pissed-off Penny is at dinner on Valentine's Day with Leonard and their friends Bernadette Rostenkowski (Melissa Rauch) and Howard Wolowitz (Simon Helberg). "We were doing this scene where Leonard and Penny get in an argument at the table, and we told first AD Anthony Rich we had a prank, and we were going to do it," Cuoco recalled to Radloff. "This would not be funny today, but it was really funny at the time—or at least we thought so—where our argument would get really heated, and Johnny would fake-punch me, and I was going to fall off camera off the chair. So we do it and it gets really heightened, and Johnny comes at me, and..."

"And she landed under the table," Galecki continued. "And then I started to pretend to stomp on her face."

Rauch added that Cucco "came up from the table with blood on her head, I thought that it was fake blood for a second." The blood was, as it turns out, not fake at all.

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As Kaley Cuoco told Jessica Radloff, nobody noticed, at first, that anything was wrong at all — they were even laughing at the well-staged prank — until she came up from the table and was bleeding quite heavily. 

Director Mark Cendrowski correctly pointed out that cuts on a person's head or face can bleed quite a lot, so it looked very, very bad in the moment — and to make matters worse, the production couldn't even work around the issue. "Any cuts on the head bleed a lot worse, but she was cut right under the eyebrow," Cendrowski recalled. "Immediately it started swelling, so we got pressure and ice on it; the makeup team even tried to cover it up, but it kept bleeding. We were just trying to joke around, but boy, we went too far with that one."

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Okay, so this all sounds pretty bad — and from there, it got worse! Kaley Cuoco told Jessica Radloff that not only was she bleeding a lot, the flow showed no signs of slowing down whatsoever. "It wouldn't stop bleeding!" Cuoco recalled. "At 7 p.m., I had to go to a plastic surgeon so they could stitch it up. The next day on the set there were all these signs saying things like, 'No more jokes! No more rough-housing!' I mean, we thought it was so funny, but no one else did. And then we had to reblock the scene on the couch after me and Leonard were at dinner, and I'm sitting facing him. Originally I was on the other side, so we had my bangs come down to cover the stitches so you couldn't see what had happened. It was so embarrassing, and the whole thing was a total nightmare."

The fact that the crew put up signs saying nobody was allowed to pull pranks or "rough-house" anymore is, admittedly, pretty funny — even though it's much more serious than some of the series' other "rules," including that the men weren't supposed to grow beards during filming — but according to Mark Cendrowski, creating the episode after Cuoco's injury was a total pain ... because the actress' battle scars were so plainly visible. Not only that, but Cendrowski said the episode eventually required special effects. "Yeah, I tried to avoid shooting her straight on when we were able to film again the next day because you could see the swelling," Cendrowski revealed, adding that any swelling was fixed in post-production. 

"The Big Bang Theory" is streaming on Max now, so go check out "The Tangible Affection Proof" and see if you can spot any weird angles thanks to Cuoco's injury.