Prospective memory research almost exclusively examines remembering to execute an intention, but the ability to forget completed intentions may be similarly important. We had younger and older adults perform a prospective memory task (press Q when you see corn or dancer) and then told them that the intention was completed. Participants later performed a lexical-decision task (Phase 2) in which the prospective memory cues reappeared. Initial prospective memory performance was similar between age groups, but older adults were more likely than younger adults to press Q during Phase 2 (i.e., commission errors). This study provides the first experimental demonstration of event-based prospective memory commission errors after all prospective memory tasks are finished and identifies multiple factors that increase risk for commission errors.