In The One with the Bullies (1996) the fire hydrant is right outside of Central Perk at the curve of the curb. In this episode the fire hydrant has been moved to the sidewalk across the street from Central Perk.
When Joey, Ross and Chandler are talking about going out to party, Chandler's bedroom door is closed at one point and opened the other.
When Monica brings the newspapers and tells Phoebe to look at the back page, Phoebe actually just flips the folded newspapers over, so she is looking on the bottom half of the front page.
The couple on the corner in the background in the opening scene miss their cue and are just standing until the gang enters the scene. Only then do they begin to walk.
Monica hands a folded over copy of a newspaper to Phoebe, saying her restaurant review is on the back page. Phoebe then turns the paper over in its folded state and starts reading, meaning she is actually reading from the bottom half of the front page and not the back at all.