When Penny married Leonard in season 9 of the character finally committed herself to their relationship by becoming Mrs Penny Hofstadter.
However, fans of the show may have realised early on that Penny's previous surname is never formally mentioned, and this creates a clear contrast to the other main characters, whose last names are regularly mentioned throughout the series. It is curious why the show writers never revealed Penny's name. But one eagle-eyed viewer spotted it was actually there the whole time.
But why was Penny's last name never officially revealed?
According to TBBT showrunner Steve Molaro, Penny's surname was intentionally kept a mystery for as long as possible. And yet when it got to season 8, which sees Penny and Leonard engaged as a couple throughout, there is still no hint of what Penny's full name might be. The suspense had likely been building for so long that picking a name that wouldn't disappoint fans became too much pressure for the writers.
Although the writers were keen not to give Penny a surname, it seems not everyone working on the show read the memo.
During a scene in episode 18 of season 2, Penny can be seen opening a parcel addressed to her that includes her full name on the label. The address can be seen as "Pasadena CA 91125", and the name addressed to is "Penny Teller".
So how did Penny Teller make it onto the box? According to Jessica Radloff's book: 'The Definitive, Inside Story of the Epic Hit Series', Molaro explains how this came about in the episode. According to him, the prop master Scott London needed a placeholder name to put on the box for the scene. Series co-creator Bill Prady gave him the name Teller as a reference to the celebrity magicians Penn and Teller. The label was never meant to be visible on camera. However, it seems to have made its way into the scene nevertheless.
Molaro states that although Teller can be seen on the package, he considers the surname as not canon. According to him, Penny does not, and will never, officially have a last name in the show (before marrying Leonard). completely agrees as she has admitted to liking the mystery it gives the character.
Whether Teller is Penny's real surname or not is maybe a question best left for fans to decide for themselves. Can it be denied if it's clearly visible in the episode? Possibly she likes to order her items under an alias. What we can be certain of is that most of the time, she is simply just Penny.