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These days, if someone "reads the riot act" to you, they probably aren't actually reading anything. They're just reprimanding you, with gusto.
The original Riot Act was a completely different thing.
In the early 18th century, the Riot Act was something actually read aloud by the agents of King George I, who used it to break up gatherings of more than twelve people by ordering them to disperse within an hour. (One practical issue: it can be challenging to read something audibly during a riot.)
The term's meaning has changed over the centuries, but it still suggests a serious offense.
For instance: "Japan's skiing officials have been read the riot act after one of their snowboarders wore his Vancouver Olympic uniform in 'hip hop' style." (Reuters, Mar. 15, 2010)