For the first time in the series, Sheldon admits he can't solve a math problem.
Bert mentions chromic acid and white vinegar to clean lab equipment. For a long time the ultimate lab glass cleaner has been chromic acid, a solution of potassium dichromate with a little bit of water and a whole lot of concentrated sulfuric acid.
In S5 E13, The Recombination Hypothesis, Leonard tells Penny on their date, that he has a government grant to create a laser for the military. Colonel Williams states the military is still waiting on a laser defense system.