When Leonard and Raj meet Nathan Fillion, they mention Dr. Horrible's Sing-Along Blog (2008). Simon Helberg (Howard) also starred in that series as Dr. Horrible's henchman Moist.
The episode won an Emmy Award for Outstanding Multi-Camera Picture Editing for a Comedy Series.
Both Sheldon and Kripke are correct in saying that Thomas A. Edison did not invent the light bulb. He did, however, create the first VIABLE light bulb. Until his efforts, light bulbs only lasted a few minutes.
It is believed (though not confirmed) that Kaley Cuoco's emotions during the scene of Howard's mother's death were more real than acting, as she was on a previous show where an actor playing a parent on that show died in real life: John Ritter on 8 Simple Rules (2002).