After she breaks Rachel's seashell lamp, Monica puts her jacket over the back of the big chair. When Chandler sits on it, the jacket keeps disappearing and reappearing in almost every shot.
When Janice is showing her engagement ring to everybody, her arm is bent about 90 degrees. However, in the next shot with the group looking at her ring, her arm is straight.
The set for Heckles' apartment clearly has been reconstructed. In The One with the Blackout (1994), when Rachel and Phoebe are at his door with the cat, the door is shown to open the left when standing inside. Here, when Joey is inside comforting Chandler, the positioning of the door shows it would open to the right.
Outside Mr. Heckles apartment and after Mr. Treeger says 'they found a broom in his hands,' the camera switches focus to Ross and Joey, and Mr. Treeger is just in the frame and his lips are moving (possibly saying the previous lines), but no voice is present to match.
When the gang and Mr Treeger watch the paramedics take Mr Heckles away, a thin dark object like a camera mark or a pencil suddenly appears on the floor close to Ross' feet.
There appear to be errors of geography with the floor plans of the "Friends'" apartment building. There is a photo of the "Friends" standing in the hall outside of Mr. Heckles' apartment on this IMDb episode entry for reference. Phoebe and Chandler are standing in front of Apt. 6, while Monica, Mr. Treeger, Ross, and Joey are standing in front of Apt. 7. Mr. Heckles' was living in Apt. 8, which is the door directly opposite of Apt. 6. From what is known of the hall and stairs outside of the "Friends'" apartments one floor up, there should be a stairway where Apt. 7 is. Since there isn't, this supports the concept that the stairs leading away from Monica's and Chandler's apartments is a short staircase with an unseen hallway extending away with an additional staircase that would lead down to the ground floor and possibly up to the floor above Monica's and Chandler's apartments. (at around 9 mins) Rachel and Monica leave to take an ugly lamp up to their apartment. About 2 minutes later, the girls can be heard on the stairs at which Chandler and Joey make a remark that it was noisy up there.
Heckles' apartment is largely implied to be directly below Monica's. When reading his Big Book of Grievances, Chandler reads entries about Joey and him, bringing home a date and dry cleaning, respectively. Unless they detoured into Monica and Rachel's apartment before going into their own, Heckles likely would not be bothered by noise by Joey and Chandler since their apartment is across the hall (likely above the girls in Apartment 5, as seen in The One Where the Monkey Gets Away (1995)).
(at around 3 mins) The gang is outside Mr. Heckles' apartment talking to Mr. Treeger while Mr. Heckles' body is being wheeled away, and a cop is making out a report. On the wall behind the cop is the shadow of the boom mic that slowly moves up the wall.
At the beginning, Joey tells the story about the 'girl' he dated with the big Adam's apple. Later in the series, he calls his Adam's apple Joey's apple, and Chandler has to correct him by telling him it isn't named after each individual person.
Phoebe asks Ross about the time when scientists believed the earth was flat. However, there was never a time when scientists believed the earth was flat. Since ancient Greek times, scientists have known the earth is spherical.