Recent evidence suggests that proactive interference (PI) does not hurt event-based prospective memory (ProM) the way it does retrospective memory (RetroM) (Oates, Peynircioglu, & Bates, 2015). We investigated this apparent resistance further. Introduction of a distractor task to ensure we were testing ProM rather than vigilance in Experiment 1 and tripling the number of lists to provide more opportunity for PI buildup in Experiment 2 still did not produce performance decrements. However, when the ProM task was combined with a RetroM task in Experiment 3, a comparable buildup and release was observed also in the ProM task. It appears that event based ProM is indeed somewhat resistant to PI, but this resistance can break down when the ProM task comprises the same stimuli as in an embedded RetroM task. We discuss the results using the ideas of cue overload and distinctiveness as well as shared attentional and working memory resources.