Human mobility prediction, a core task of location-based web services, forecasts a user's next Point of Interest (POI) visit from historical trajectories, supporting applications from recommendation to urban planning. A general perception in the literature has been that visits to popular POIs are relatively easy to predict. Our analysis challenges this perception and shows that, even for visits to popular POIs, many predictions fail. The underlying cause is often transition-level sparsity, where a transition is a move from a source POI to a destination POI. While a POI may be popular, its transitions from other POIs may still be very sparse, since there are many POIs in a dataset. As a result, predicting a popular next POI can still be challenging if the transition from the source POI to it never appears in the training data. We therefore argue that a core bottleneck in human mobility prediction lies in the transition-level long tail, and propose REtrieval Combined with pArametric memory for next-POI prediction (RECAP). RECAP learns frequent transitions in the parameters of a small Transformer trained from scratch. Long-tail transitions give the parameters little or no training signal. RECAP retrieves evidence for them from two sources: (i) multi-hop transitivity learned from a global transition graph of all users, and (ii) revisit evidence from a target user's historical trajectory. RECAP further uses two-stage training to let the revisit component also learn from frequently seen transitions. Experiments on three real-world check-in datasets show that RECAP improves prediction accuracy, with clear gains on tail transitions. Our code is available at https://github.com/NiallLDY/RECAP.

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