First appearance of Amy Farrah Fowler (Mayim Bialik). The character is loosely based on Gilda, a character that was cut after the failed first pilot of the show.
Leonard estimates the height of the lift shaft by timing how long it takes until he hears breaking glass. This is done by multiplying the number of seconds by itself and then multiplying by 16 (to get the height in feet) or 5 (to get it in meters). Given that the time is about a second and half, Leonard's estimate of 30 feet is a reasonable one, which isn't bad, given that he is drunk at the time.
Penny mentions Sheldon's noise-canceling headphones to help him sleep while she and Leonard have sex. Sheldon introduced these himself in The Plimpton Stimulation (2010).