Okay, take a look at this: http://viscog.beckman.uiuc.edu/grafs/demos/12.html

I have seen other demos of a similar situation. If you were in the same situation, do you think you'd notice the change?

Think about it seriously for a moment.

The results have varied, from around 25% to as low as 5%. I believe there was one experiment where not one of the 42 participants noticed a change (but I'd have to dig for that).

Our cognitive capacity is clearly limited, and a lot more limited than most people believe. So, in a sense, the fact you filtered out the monkey was actually an asset. It was irrelevant additional information, and this accurate elimination of irrelevant information is one problem AI faces.