(1) local zoning authorities can change their minds and take steps to gradually cancel the nonconforming use over time;
(2) you can lose the right if you substantially change the existing use of the business (i.e., a garage becomes a Blockbuster video franchise), your building is damaged or destroyed (i.e., your garage has to be rebuilt because of a fire), you shut down operations for several months (the length of time varies with local law), you discontinue operations entirely, or you significantly expand your business operations (i.e., your garage now includes an auto parts store).