Prospective memory refers to the ability to remember to execute future intentions (e.g., taking medication with dinner). Although most prior research on prospective memory errors has focused on omission errors (i.e., failures to perform an intention in response to a target cue), there has been a recent surge in research on commission errors, the erroneous performance of a finished intention. Existing studies have examined factors at retrieval that lead to commission errors; the present study extends this research by investigating the impact of encoding strength. We found that relative to standard encoding, implementation intention encoding doubled the risk of commission errors in our laboratory paradigm for both young and older adults. This novel finding demonstrates the impact of encoding strength on commission errors and documents the potential challenges of deactivating the effects of implementation intentions upon completion of a prospective memory task.