13 The Magna Doodle on Joey and Chandler’s apartment door has a different drawing in every episode in which it could be seen. 

The doodle isn’t just always different, it also seems to contain some kind of message. The Magna Doodle board was first introduced in the episode “The One With the Hypnosis Tape” in season three with a message that says “Joey, call your dad”. Several times the doodle contained cartoon faces, or sometimes smiley faces. In the episode “The One Where Chandler Crosses the Line” in season four, Treager had left a message that said “Clean up the duck feathers in the hallway! Treager”.(source)

14 Originally, the show was to be focused on four characters, with Phoebe and Chandler in supporting roles. The creators toyed with the idea of Chandler being gay, but changed their mind after they met Matthew Perry. However, they did insert many jokes about Chandler having gay mannerisms. 

15 James Michael Tyler, who plays Gunther, was hired because he was the only extra who knew how to use an espresso machine. He is also the only supporting member of the cast to have appeared in over 100 episodes, 151 to be exact. 

16 In the series, Monica says the first lines and Chandler says the very last lines. 

17 In the opening credits of season six episode “The One After Vegas”, all the names had “Arquette” added at the end as a gag on Courteney Cox Arquette who had recently married David Arquette.

18 Jennifer Aniston almost lost the part of Rachel because of her commitments to another series called “Muddling Through”. She was allowed to film Friends under the condition that if Muddling Through became a hit, she’d pull out from Friends pilot.

When the casting process for Friends started, Jennifer Aniston was working on “Muddling Through”. So, during that time she had to stand out of the group photographs because of the uncertainty. Some of her friends would phone her and tell her that they were auditioning for her part. So, for a period of two weeks she had to travel between Sony and Warner Bros. studios. But, in the it all turned out well as “Muddling Through” was cancelled as expected, and she joined the cast for good.(source)