The detective that comes into the bar looking for Brian LeFeve is the same detective from series 2 on the episode The Gang Go Jihad.
In the Fight Milk promo video, Charlie and Mac use the dick flyer from S03E03, "Dennis and Dee's Mom Is Dead", on the fight milk bottle.
When Frank is berating a subordinate, recycling bins next to the photocopier have the CBS "eye" logo.
In Charlie and Mac's promotional video for Fight Milk, Charlie is seen wearing a Merrimack College lacrosse shirt. In real life, Charlie Day graduated from Merrimack College with a Bachelor of Arts in 1998. In 2014, he gave the commencement speech at Merrimack, which is when he was honored with a Ph.D.