The Big Bang Theory: 21 Easter Eggs You Probably Missed
6. Smurfs Photo
During The Holographic Excitation in Season 6, the gang debate over what to wear to their Halloween party, with Howard and Bernadette opting for a pair of fully-painted smurfs.
A snapshot of the couple in those costumes shows up later in the series to highlight that the memories being made are actually happening. There are other reminders of previous parties and event scattered within the show.