Over on the Right Coast, Mike Rappaport has a nice post on Hayek's description of common law judging. This is from the middle of his post:

. . . [Don] Boudreaux gives the example of a high school cafeteria, where individuals save their seats by putting their books down on a seat before they get in line for food. When they come back to their books, they reasonably expect that their seat will be "saved." If someone had taken their seat, pushing their books aside, it would be the job of the common law judge to enforce their reasonable expectation that seats could be saved.

This is a good example to illustrate Hayek's conception of the common law, and it can be developed further than Boudreaux and Roberts do on the podcast to illustrate some other aspects of Hayek's theory. First, Hayek claims that judges are limited to enforcing rules which can be known in advance, but this might be thought to be in tension with enforcing an unwritten law. This example shows that there need not be a tension. If judges are enforcing existing customs, they can enforce unwritten rules, without "making up" the law. Second, Hayek also says that judges are not making policy. Here, judges are simply enforcing the customs that have developed. Third, even though judge are not making policy, Hayek imagines common law judges as enforcing rules of conduct that should promote desirable outcomes. Because Hayek believes that the customs that emerge tend to be desirable within an order, the enforcement of those customs and expectations will result in desirable rules, even though judges do not directly aim at developing a desirable rule. . . .

Hayek's account from Law, Legislation and Liberty is important to appreciate the difference between common law judging and policy making.