Roy's shirt displays the acronym "RTFM" above a picture of Chairman Mao holding up a book. RTFM is a popular acronym in IT that means "Read the F*cking Manual", since many problems encountered by those who call tech support could be solved easily if the customer would only take time to read the instructions that came with the device. Related acronyms include PEBKAC (Problem Exists Between Keyboard and Chair) and I.D.-10-T (idiot) Error.
It's established that Moss carries around a spray canister of water to cool his ear when it gets hot. That trait was dropped from the entire show's run afterwards, but it came back one more time in the final episode of the show.
Behind Roy's desk is a standalone toy often confused with Stewie Griffin from Family Guy. It is actually Jimmy Corrigan, a character from a comic series predating Family Guy. On the wall behind Maurice is a sketch of The Flying Spaghetti Monster.
The gag with Reynholm talking into the mouse of his computer to try out the "new voice activation system" is a reference to Star Trek IV: The Voyage Home (1986), where Scotty does the same thing when the Star Trek crew travels back in time into the 20th century.
Jen shows her poor command of language when she says "You're used to being social piranhas". A piranha is a small voracious fish. A pariah is a social outcast, not merely disliked but someone to be actively avoided.