Automated program repair (APR) techniques have received much attention and APR tools have promising results in repairing real-world bugs, but what are APR’s limits? Knowing the answer to this question could help researchers design trade-offs for APR tools and manage user expectations. Since APR as a whole is undecidable, as are two of its typical phases, one must use conservative approximations. Conservative approximations can help APR tools be better understood and can lead to a theory of sound APR.