In the scene where Amy and Bernadette return to the hotel room drunk, the amused reaction of Kaley Cuoco (Penny) to their drunken antics is genuine.
At the start of this episode, Sheldon mentions that they never made any progress with their electromagnetic energy idea that they had on 22nd September 2007 because two days later, Penny moved in. The series indeed started on 24th September 2007, opening with Penny moving in in Pilot (2007).
While white-boarding the Back to the Future Part II (1989) timeline, the boys have trouble with verbs to describe actions that have been altered by time travel. This parallels the Red Dwarf episodes Future Echoes (1988) where Lister and Rimmer have a similar discussion about how to conjugate verbs to describe a future event that has been erased by time travel, and The Inquisitor (1992) where Kryten refers to the so-called 'past impossible never tense'. All this was most likely inspired by Douglas Adams's sci-fi parody novel 'The Restaurant at the End of the Universe' (sequel to the often mentioned 'Hitchhiker's Guide to the Universe') which mentions 'The Time Traveler's Handbook of 1001 Tense Formations' for every conceivable time travel situation (and adds that most readers never got beyond the Future Semiconditionally Modified Subinverted Plagal Past Subjunctive Intentional).
On the Chuck Lorre Vanity Card #463 shown last episode, he stated that that would be his last vanity card. However, he wrote #464 which is shown in this episode.
This episode saw Howard Wolowitz roll up his sleeves for the first time in the entire series.