Yes there is that, but there is a difference between doing it on purpose for a character, and doing it just because it's habit. And the difference can often be quite obvious - as I was told in an exam when I was younger. If you're aware of it because you're doing it on purpose, you can still make the words coherent, whereas habit can affect your clarity

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Terry Pratchett to Fleur - "Oh, are you the cleavage lady?"
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