CommonSenseIsnt is uttered, muttered and occasionally stated outright by those experiencing a certain degree of frustration with other people's levels of (in)competence. We frequently assume, often implicitly, that a user, customer, whatever, will exhibit a certain degree of CommonSense, only to find that they do something so extraordinarily unexpected that the only possible conclusion is that they have no sense at all.

See also CommonBloodySense
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Very rarely am I asked "Isn't what?", which indicates to me that language is an amazing thing.

''I guess they are confused whether CommonSense isn't Common or isn't Sense? Both might apply.''

Not only that, but this is a form of word play; it's not a grammatical sentence until you spot the intent that it is being self-referential and one should take the missing end of the sentence to be the beginning of the sentence (or equivalently, are previously familiar with such forms), so it initially sounds like you're trailing off "XYZ is not...''Yes? Go on...''"

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It seems to take about 10 years for kids to start developing common sense. Maybe longer.
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See also FourLevelsOfCompetence