Penny's dress in the Nobel Prize ceremony from The Big Bang Theory finale featured a secret and touching Easter egg. 

The Coopers thanked their friends during the event, and since it was a formal occasion, everyone dressed up nicely. Penny wore a black dress with a pink cummerbund, which apparently included a Penny Blossom — an accessory that she created as a business venture back in The Big Bang Theory season 2 as Mary Quigley reveals in Jessica Radloff's book, The Big Bang Theory: The Definitive, Inside Story Of The Epic Hit Series. Read her full quote below:

"Mary T. Quigley: I incorporated Penny Blossoms in Penny's dress… slightly hidden in the cummerbund, if you will—in that hot pink sash. It's a secret that Kaley and I knew. We both got teary when we were in the fitting, because we got to do Penny Blossoms again. "

The addition of Penny Blossoms in Penny's Nobel Prize Ceremony dress is a great way to pay tribute to her overall arc in the show.