When Leonard says he's going home for a nap, Sheldon tells him he can't go because he has to take him to the dentist at 4pm. However, when Sheldon arrives home later that evening he informs Amy that she will be taking him to the dentist the next day. It is never mentioned that the appointment had been rescheduled.
Sheldon creates "Leonard's Day" as a solution to their current dilemma; a day to recognize everything Leonard has done for him. From here on, there's never been a "Leonard's Day".
When turning off his Bunsen burner, Sheldon reaches down below the table (presumably to turn the gas supply off), then deliberately blows the flame out. This is pointless. Turning off the gas kills the flame, and there would be nothing to blow out. Indeed the flame should have died the moment he turned the gas tap.
During the blackout, Sheldon tells Leonard that he has all 61 episodes of Red Dwarf (1988). At the time of this episode's airing there were only 55 episodes of Red Dwarf.
It is read that Howard will be going up with Tom "Tombo" Johnson and Mike "Supernova" Novoselic. He ends up going up with Michael "Mas" Massimino and an unnamed Russian Cosmonaut.
Raj suggested "Buzz" as Howard's space name. When Howard rejected it, he cited Buzz Lightyear (from Toy Story (1995)). Being an astrophysicist, it's very unlikely that Raj is unfamiliar with Buzz Aldrin.