Where do you put your attention?
It appears that some people don't notice the switch and while some take a second look they continue on with the conversation.
This came to my attention through a podcast on PRI's The World discussing a new book that describes how scientists are studying the art of conjuring and deception to better understand how the mind works.
Dr Stephen Macknik, one of the books authors, explains why people miss the "people switch" shown in the video:
The fact is that this effect works not because these people don’t pay attention well, but rather because they pay attention very closely ***to the task at hand***, which is giving directions. When you pay attention to something, we’ve shown in our research that everything else gets suppressed in the brain.
From a time management point of view it might be interesting to notice if, when you are focused on a task, you are ignoring the context, i.e. its priority, its urgency, its value.
You can find more about Dr Macknik's work at his web page.