Bryan Caplan says that maybe liberals and libertarians can agree that farm subsidies are bad. However, I am not impressed by this. Liberals have no problem reducing government support for groups they dislike, such as "corporate farmers." But they still want the government to support groups that they like--locavores, or what have you.
I think that (non-classical) liberals and libertarians see the problem of "special interests" differently. Liberals view special interests as exogenous to the policy process. You have to overcome special interests to create good policy. Libertarians see special interests as endogenous. Policy is what creates them.
I saw this last week in a discussion of energy subsidies. Liberals have no problem if you want to get rid of subsidies to oil companies. But they cannot really accept the notion that "green energy" deserves no subsidy. What's not to like about wind and solar?
Most important, liberals will always say that we need government involvement in food policy, energy policy, education policy, health policy, etc. When they observe that a particular policy serves only special interests, they may appear to side with libertarians by supporting a rollback of the offending program. However, while the libertarian will put forth the notions that public policy is often self-defeating and that it is impossible for policy to be immaculately removed from special interests, the liberal is never going to concede those points.