'Big Bang Theory' Says Goodbye: 16 Behind-the-Scenes Secrets From the Emotional Finale Taping

Warning! There are some plot spoilers hidden among these on-set observations...

You may have already watched The Big Bang Theory’s tear-jerking finale, but you definitely didn’t see these emotional behind-the-scenes moments!

ET was on set in Burbank, California, for the live taping of the CBS comedy's very last episode and when we weren't freaking out over all the hidden Easter eggs or cheering for the surprise appearance of Sarah Michelle Gellar, we were holding back tears while watching the cast say goodbye to the characters they've played for the past 12 seasons.

Here are 16 on-set moments that every Big Bang fan should know about...

(Warning! There are some plot spoilers hidden among these on-set observations.)

1. Hours before the show started filming, stars Johnny Galecki, Kunal Nayyar, Simon Helberg and Kaley Cuoco surprised the fans camped out in the Warner Bros. waiting area who were hoping to snag seats to the finale.

“Y’all are the greatest support any cast could hope for and we love you dearly,” Galecki posted on Instagram along with a video. “And if you don’t get in tonight, trust that you are in our hearts during this last performance.”

2. The live studio audience was packed with a mixture of cast and crew family members, friends of the series, press and mega-fans of the show -- many of whom were sporting matching T-shirts.

3. Cuoco's husband, Karl Cook, was standing with a smile on his face at the back of the live studio audience. He stayed for the three-hour taping and during the final scene, he was seen wiping tears from his eyes.

4. Jim Parsons’ mom, Judy Parsons, was seated in the front row of the audience. After Big Bang’s audience MC introduced her to the crowd, Judy happily stood up, clearly beaming with pride, and waved to the rest of the fans in the audience.

5. Cuoco's sister, Bri Cuoco -- who was the choreographer of the Big Bang’s epic flash mob dances to “Call Me Maybe” in 2012 -- was also in attendance. (Cuoco Family Fun Fact: There have been 279 episodes of Big Bang, and Cuoco’s dad, Gary Cuoco, has never missed a taping!)

6. The Big Bang Theory’s costume designer and co-producer, Mary T. Quigley, and many other women on set were sporting sparkly crowns during the taping, which was a sweet nod to Amy Farrah Fowler’s infamous freakout over her tiara.

7. Many of the cameramen -- who typically sport a very casual wardrobe while filming -- stepped up their fashion game in honor of the finale and wore nice blazers and slacks while operating the high-tech equipment.

8. It was a Big Bang birthday bash! The day of the finale taping, April 30, also happened to be Galecki and Nayyar’s birthday. To make the celebration even sweeter, the crew also got a cake for co-star Melissa Rauch.

"You know you work with wonderful and extremely thoughtful people when they celebrate your June birthday along with today’s birthdays because you won't be together when it comes around this summer,” she wrote on Instagram.

9. Emotions within the cast were very high from the get-go. When the audience MC first introduced each member of the cast, the tears were already flowing as the stars waved to the crowd.

10. During the scene when Leonard pulled Penny aside and asked her, “Was Amy suspicious when they had to let your dress out?” the studio audience collectively gasped as they began to put the pieces together that Penny was pregnant. The screams and applause were so loud by the end of the scene, that the director had Galecki and Cuoco film another take for safety.

11. After the quick, but sweet scene with Sheldon and Amy telling the group how to behave while in Sweden, the whole cast came together for a long group hug. That was the very last scene in The Big Bang Theory’s iconic living room set.

12. At one moment during the taping, Galecki and Cuoco climbed up on the railing of the audience bleachers to say a few words into the microphone directly to the fans. "This is unbelievable. It’s been an honor,” Cuoco said through thick sobs. “Thank you so much for supporting us and for being there from the beginning. You've been unbelievable letting us make you laugh for so long... The Big Bang Theory will live on in our hearts."

13. While the fans waited for the crew to finish setting up one of the last scenes, the audience broke out into a sing-a-long of Sheldon’s beloved childhood lullaby, “Soft Kitty.”

14. In another touching moment toward the end of the episode, Big Bang creator and executive producer Chuck Lorre struggled to hold back tears as he marked the start of one of their last scenes with the slate.

15. In the following scene, Galecki and Cuoco were both crying and Galecki actually had to stop mid-scene because he was too emotional to say his lines correctly.

16. During the curtain call and final bows, Mayim Bialik took the lead as the cast walked out together while holding hands. Rauch, Galecki and Cuoco were all crying the hardest while the rest of the stars were struggling to hold it together. The audience and the crew gave the cast a long standing ovation.