Corrected entry: About 1/3 through the episode, they are in Roz's apartment. If you watch when people enter or leave the apartment, the door has a handle lockset. It also has a lock cylinder about 1 1/2 feet above it on both sides of the door. The problem is, the side of the door has no drill hole where the latch assembly should be: it's just solid wood. The lock isn't attached to anything that would enable it to work.
terry s
15th Oct 2011
Correction: This is not a mistake. Often tenants install deadbolts and vacate without leaving a key, and it is quite common for the landlord to just remove the bolt assembly and patch the edge of the door. By leaving the lock in place, the landlord avoids having to replace the door, or repair a large hole through the door with 2 finished surfaces. If Roz had actually used a key in the lock, THAT would be a mistake.