The One With The Flashback - S3-E6
Corrected entry: Just before Monica invites Joey in for lemonade, she says,"It's the heat. And the humidity." When she says this, if you look between the doorframe and the edge of the door, there is a crew member behind the door moving around. (00:11:00)
Correction: They live in a building with several apartments. The person in the hallway could have been another resident, a visitor to someone living there, a janitor or any number of people with a legitimate reason to wander around inside the building.
Twotall