Penny: What the hell is that?
Leonard: What was it doing on the ceiling?
Wolowitz: That's classified.
After Sheldon mentions a molecular sieve, they go over to 4a, their apartment. 4a is a kind of molecular sieve. See more...
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Deliberate "mistake": Very often, during Skype or other video chat conversations, the people on the computer screen look left or right when talking to someone who is not directly in front of the screen. Although it certainly helps the viewers understand who's talking to whom (which is why I assume it's a 'deliberate mistake'), it isn't realistic, since it implies that the people on the screen are actually constantly looking away from their computer instead of at their screen or webcam.
Continuity: In the apartment block's stairwell, the wall mounted light is situated in a different place in the two camera angles used. In the first angle showing them walking down from behind, the light is positioned above the level section of hand rail, when the camera switches to the front view, the light is over the angled section of hand rail.