What would The Big Bang Theory be without Kaley Cuoco's loveable blonde Penny? Thanks to the CBS comedy's unaired pilot, viewers get a small taste. In the 2006 episode, which was not picked up the network, Penny's character is replaced by another blonde named Katie.

Sheldon Cooper and Leonard Hofstadter, still played by Jim Parsons and Johnny Galecki respectively, come across a crying Katie, played by actress Amanda Walsh, after she is dumped by her boyfriend. Leonard invites Katie to join them for dinner and stay at their apartment for a couple days — despite Sheldon's objections. Fast forward to the end of the episode, and the gang is having an awkward dance party after Katie agrees to live with them.

Another major difference between the unaired pilot and The Big Bang Theory of today is Leonard and Sheldon having a female friend instead of hanging out with Howard Wolowitz and Raj Koothrappali. In their place is nerdy Gilda, played by Iris Bahr, who clearly has a crush on her "research partner" Leonard.

"Let's put our cards on the table, shall we," Gilda tells Katie in the episode. "Right now, Leonard and I are doing research together. It's casual, professional. A couple of months from now, there'll be a transition. 'Hey, what about a cup of coffee?' Which of course will be followed by the initiation of sexual congress, social coupling, offspring, etc. That is unless some female wants to challenge my position. "

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Fans also noticed major character differences, including Sheldon and Leonard's more "old-fashioned" style compared to their signature comic-book inspired wardrobe and Sheldon is uncharacteristically sexual — talk about Bazinga. So why didn't Katie make it onto the network? She was reportedly cut due to audience surveys calling her "too mean" and creator Chuck Lorre decided to add two more male characters to the cast, at the expense of Gilda.