I have recently begun watching The Big Bang Theory. Mostly passively, this is my wife's new show. For this reason I have noticed some things, but haven't watched closely enough to notice small differences.
I wonder about the fact that each of the floors look the same as they walk upstairs. Researching further, I see that the three floors (including the one they live on) with two staircases and a warning-taped elevator are all actually the same set (see item #2 in this list).
This site does reference the fact that they make small changes to distinguish each of these three floors. Does anyone know what these differences are? Are they listed somewhere on another site?