Continuity mistake: After Jack brings the puppy over to the girls, the box he brought the puppy in was placed on the back end of their couch. At one point, it is knocked over, and you can see it falling down to the floor (past Janet). In a few shots, it's back on the table, and none of the characters were in a position to pick it up to place it back there.
Continuity mistake: While Jack is talking to the police officer about the missing rent money, they are standing in front of the TV set and the front door is open. When it cuts back to Jack and the officer, over Jack's shoulder the front door is closed, but when the officer turns to leave the door is open again.
Continuity mistake: When Jack is introduced to Mr. Roper for the first time, the girls just say he's their landlord (no name given yet), however after Jack's talk in the kitchen with Chrissy (in which still no landlord name given), Jack storms out and starts out shouting "Look here Mr. Roper..." He still should not have known the landlord's name at this point.