One of the most important locations in Friends was Monica's apartment, but the one she lived in actually changed numbers, as did that of Joey and Chandler's - and here's why. Friends is packed with minor details, and fans have noticed the numbers on the doors changed through the seasons. While this can be easily attributed to a continuity error, there’s a reason behind it.
Friends, which turns 25 this year, had a number of episodes and key moments set in these two apartments, which alongside Central Park served as the most common and important locations in the series. The sets in Friends, especially the aforementioned apartments, were packed with details that changed throughout the series, some due to plot reasons, others were continuity errors, and others like the numbers of the apartments had to be changed due to logic.
Monica, Rachel, Chandler, and Joey lived at 90 Bedford Street in New York City, and in the first season of Friends, the number of Monica’s apartment was 5 and Chandler’s (then Joey’s, after Chandler moved in with Monica) was 4. Nothing spectacular or weird on that, except that it didn’t match the view from the windows – especially the one they used to spy on Ugly Naked Guy. The apartments should have been higher in order to match the view (and staircases, which were also frequently shown), and so the production crew changed the apartment numbers to 20 and 19.
The apartments and the view were now in sync, but the Friends crew let other errors slip that had to do with the apartments, specifically Monica’s. The view from her kitchen window was constantly changing, going from a brick wall to a brick wall and a window, a washing line, a couple of buildings a bit far away, and a cardboard wall in the last episode. Viewers never got to see through the window in Joey’s living room (except when he showed the rest that it was snowing on Phoebe’s wedding day) as he always had the blinds down, so Monica’s apartment was the reference for the crew when changing the numbers, although they didn’t pay much attention to the other window in the kitchen.
With a show as big as Friends and with an art department as detail-oriented as the one they had, it’s interesting that they paid attention to something as apparently simple as the numbers in the apartments and the views, but it’s also understandable that some continuity errors slipped, which only adds to the fun of rewatching the episodes.