First of all, 'case in point', 'for example' and 'for instance' are not words – they are phrases.
'Case in point' is usually used as part of the text, with 'a' before it.
'Some dogs can be vicious; the rottweiler is a case in point'.
At the start of a clause or longer phrase, 'case in point' without the article – meaning 'for example/for instance', is informal, I'd say.
'Critics are sometimes scathing; case in point – Ellie Mentary, who hates Sherlock Holmes stories'.
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