Magna doodle says "CB + MG 4Ever"
In The One Where Everybody Finds Out (1999) Ross visits Ugly Naked Guy to try and convince Ugly Naked Guy to let him rent the apartment. The number on the door is 201, in this episode the number has changed to 3B.
When Ross and Phoebe are eating snacks after being kicked out of Howard's party, you can clearly see that the C and T have been colored in with a red marker on Phoebe's bag of cheese crackers to avoid using the Cheez-It brand on air.
Rachel mentions that Monica is almost 30. In reality, Courtney Cox was already 30 by the time Friends first went on the air.
Ross (David Schwimmer) is called "3B" by the other residents of his new building. Schwimmer played Josh '4B' Goldstein (nicknamed after his apartment) on NYPD Blue (1993).