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Super Meat Boy comes to mind: https://news.ycombinator.com/item?id=3387628

Final Fantasy 14 had raw SQL queries in it's UI, so handing in a quest basically made you send an UPDATE statement straight to the DB to update your character with the rewards.

Amazons New World was a perfect example, you could literally send the server info that you're invulnerable and you would be... unkillable. People flying, telling the server they have x gold, zero validation whatsoever. Online games are a constant balance between what can be handled by client and what should be handled on the server but most of them display absolute lack of after-thought given from developers and/or management.