How does the actual incidence of a government expenditure program differ from the legislated intent?

In what ways may the actual incidence of a government expenditure program differ from the legislated intent? Why are the effects different in the short-run and different in the long-run? What examples can I use to show the effects of government farm programs. How can I explain how the short-run and long-run effects of a regulatory program, such as rent control, may differ? I'm totally lost on this and need some intelligent, attractive, wonderful person to lend a hand. Any help would be appreciated. Thank you.