Why does the sum 7×8 catch people out?

Is 7×8 the hardest sum to quickly answer?
BBC News – Education & Family

Children who start school with poor maths may never catch up

Children who start school at five years old with poor maths skills are often doomed to failure, struggling to keep up in class and never able to regain the ground lost at such an early stage.

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Lagging pupils ‘don’t catch up’

Just one in 15 pupils in England starting secondary school “behind” for their age, gets five good GCSEs, official data shows.
BBC News – Education & Family