When Rachel is watching TV with Marcel and he, then, runs out the door, two different TV sets are shown.
After Rachel has finally told Ross that she lost Marcel and the shot zooms out to see Marcel on the balcony, we return to the inside of the apartment. In one shot, we can see the wine that Ross opened and re-corked a few moments ago. However, the shot now shows the wine as if it had never been opened. Then, a few shots later, the bottle of wine has completely disappeared from the table.
According to this episode, there are four floors to their apartment building. In The One Where Chandler Can't Remember Which Sister (1997), Monica and Rachel have a neighbor directly above them. According to The One Where Heckles Dies (1995), Mr. Heckles lives directly below them. That would put Monica and Rachel's apartment either on the second or third floor and Mr. Heckles' apartment on the first or second. This means in this episode, if Monica and Phoebe are searching for Marcel on the third and fourth floors, it should have been Chandler and Joey who approached Mr. Heckles' door, having been searching the first and second floors.
When confronting Mr. Heckles with the crate of bananas, the apartment door number goes from 9 to 8.
When Monica is reading Rachel's country club newsletter, she folds back the cover. While she and Rachel are talking it's wide open and then she folds back the cover again near the end of the shot.
Mr. Heckles is clearly a thin, older man, yet the hands that reach for Marcel in the hallway are those of a fatter, younger man.
When Rachel leaves the apartment to take out the poo Marcel the monkey had done in Monica's shoe, she turns right to go to the communal rubbish room, while at other times when the gang have taken out pizza boxes, they have turned left.