In the opening scene Chandler goes into the bedroom to pack his speedo, when he returns he has a different haircut.
When Joey and Phoebe call Monica and Chandler to tell them there is a gas leak in their apartment, the items on the table change. Sometimes there is a glass bowl that Joey ate out of, and sometimes there is a decorative bowl with vegetables in it.
When Rachel walks into Ross' apartment to tell him the news, she folds her magazine and tucks it under her arm. Every time the camera angle changes while they are standing and talking, the magazine goes from being folded front cover out to back cover out over and over again.
While Monica and Chandler are on their honeymoon, the front door at their apartment is locked. Yet with the exception of the Underdog incident, the door had hardly ever been locked before. In fact, when Monica and Chandler agreed to move in together and Chandler tried to stage a moving-in ritual by going outside and locking, then opening the door, he commented that the door "hadn't been locked in five years".
When Joey and Phoebe call Monica about the supposed gas leak in her apartment, they are pretending that they need to break down the door to enter it. But they are calling from the apartment's phone and Monica would have seen her home phone number when she answered her cell phone and known they weren't telling the truth.
Mr. Treeger tells Phoebe and Joey that Monica and Chandler will know about the new door because he'd have to put a new lock on it. If one looks at the broken door in the background you notice that the lock is not damaged meaning he could simply remove the lock and put it on the new door.
Phoebe and Joey didn't consider asking Ross if he had a spare key to Monica and Chandler's apartment.
Mr. Treeger said he'd have to give Monica and Chandler new keys, for the new door. An experienced building super attendant would move the old locks onto the new door, also Monica's ornamental frame.