During the 12 seasons of The Big Bang Theory, Penny's last name was never revealed, and showrunner Steve Molaro had intended to keep it that way. However, a fan on Reddit spotted her last name "Teller" on a shipping label in season 2, which caused some buzz online.
The show decided not to confirm or deny this, leaving it a mystery. In the final season, there was a major change made to Penny's character when she became pregnant, despite Kaley Cuoco originally not wanting her character to have children. This plot twist was made to honor Leonard and Penny's relationship, which was a significant part of the show. However, Penny's last name remained unrevealed until the end.