Why do people say 'The quiet ones are the worst'?

Just out of curiosity...
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+1 y
Please note: A quiet and a shy person are not the same.
A shy person may suffer from social anxiety and confidence issues, where as a quiet person can also be sociable and fun but prefer not to be in the spotlight.

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+1 y
From what I gather it can be seen as double edge sword 🤔
Why do people say 'The quiet ones are the worst'?
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