Analysis of what might go wrong in a housing allowance program, stressing the danger of favorable experimental findings proving illusory during a national program, or flaws in program operation not anticipated by the experiment. Possible causes of experimental failure are (1) collapse due to politics, the media, or corruption and its exposure; (2) experimental errors such as careless site selection. Overadministration may lead to false acceptance of the program: special procedures may prevent underreporting; counseling may be too good; too many inspections will be possible. A national program could fail because the experiment was over-administered; because findings reflected research attention not possible in a national program; because relevant mobility and desegregation data were not gathered; because of unfavorable political or media reactions; or because of difficulties in writing and passing the law. The experiment might, in fact, substitute for analytic thought and careful social programming.