Bob turns 21 and goes out and buys his first six-pack of beer. The next week, the drinking age is raised to 22 and the sheriff arrests Bob for buying that six-pack. Bob is the victim of

ex post facto law

Explanation:

Based on the information provided within the question it can be said that in this scenario Bob seems to be the victim of an ex post facto law. This is a type of law that changes the legal consequences that were put into effect in the past, before this new law was introduced. Which is why Bob is being arrested even though at the time of purchasing the beer, he was doing it legally.

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