When Howard and Raj are rehearsing their music in Howard's living room, on the bookcase behind them are the Howard and Bernadette dolls that Howard made in The Cooper/Kripke Inversion (2013).
The exothermic "hot ice" liquid is a supersaturated solution of sodium acetate. This stuff is reusable, just reheat it to make it liquid and allow it to cool off. Side point: on the table is a small bottle of vinegar. You mix vinegar and baking soda to make sodium acetate.
The Far Side comic panel that Sheldon and Amy refer to is a real one that features a child pushing a door that has a large sign that says "Pull." There is a sign in front of the building that says "Midvale School for the Gifted."
This marks the last mention of Howard and Raj's band 'Footprints on the Moon'.
The third time that Penny uses Sheldon's knock on his own door: the first time was an angry variation in The Panty Piñata Polarization (2008). In The Communication Deterioration (2015), Penny tried again, and Sheldon noted that by the third knock, she must have realized that there is something strangely enjoyable about it. Here, Sheldon flat out tells Penny that it is annoying when *she* does it.