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)