I found an interesting case, where a water course is mapped as diverging into an underground/overground pipeline which then routes onto another water course: WaterwayMap.org - End points.
The pipes are mapped as Way: 151069423 | OpenStreetMap, Way: Conduta de água (225455540) | OpenStreetMap and Way: 225455539 | OpenStreetMap, and their direction is correct. Shouldn’t the start of the pipeline not be considered an end for the water course?
Edit: here’s another one that I’m a bit puzzled by (also involving pipelines, this time one that diverges and returns to the same water course).