Season 4, Episode 3 - A big ol' smiley face. Probably drawn by Joey after Rachel told him about the half-naked Chandler she found handcuffed to a filing cabinet at work. Or Chandler, because hey, he was getting laid for once.
Season 4, Episode 7 - Treager leaves a message which reads "Clean up the duck feathers in the hallway! Treager." Later on, it shows "I love you man!", maybe from Joey to Chandler, foreshadowing the test their friendship would endure in the next episode...
Season 4, Episode 16 - Ross is interrupted writing: "Joey eats little v.", which chimes with Joey becoming a veggie for Phoebe's sake. Later it says: "Emily...Bon Voy see ya" as it was Emily's farewell party.
Season 4, Episode 17 - First it reads "Clean bathroom" (self explanatory) and later on, "Knock, knock who's there? PORN!" Of course, this references that time Joey and Chandler found a free porn channel on TV. Oh boys, in 2016, that's called the internet.
Season 5, Episode 4 - "Evil Joe" is written on it with fire flames drawn underneath - a reference to his job at PBS, a company Phoebe hates because it reminds her of her mother's suicide. Also a reference to Phoebe's notion that no good deed is selfless.
Season 5, Episode 8 - This is The One With All The Thanksgiving Flashbacks. Number 327 can be seen inside a heart with wings and "bye" is written underneath. Thanksgiving takes place on the 327th day of the year.
Season 5, Episode 9 - Saturn and stars are drawn, which could signal Ross temporarily losing his mind, and living on another planet in The One With Ross' Sandwich (OK, we've got nothing. You got any better ideas?!)
Season 6, Episode 3 - There's an island with palm trees on it, a shark in the water and a man sitting in a boat. Ross can't come to terms with his feelings for Rachel - could this be how he feels inside?
Season 6, Episode 20 - A robot head, referencing Joey's part in sci fi show Mac and C.H.E.E.S.E. Later, Chandler writes to Joey: “Mac audition at 2:00 allergy actor attacked.” He later adds: “By dog, not flowers.”
Season 8, Episode 10 - It reads FDNY beside a drawing of the Statue of Liberty - most definitely a reference to the New York Fire Department, who worked tirelessly to keep the city safe during terrorist attacks at that time.