I encountered this error and accompanying funky behavior when simply creating vertices to extent a line and/or line segments. Very simple - existing feature --> edit --> edit vertices. Then begin adding vertices to an existing polyline feature class. In this case, a feature class with 4 features. To solve this, I kind of followed an ArcMap Error that suggested Properties --> [Rebuild Spatial Index] ESRI Error and Solution . In Pro (v3.02) go into Feature Class Properties --> Indexes --> expand "Spatial Index" --> Recalculate. Solved. Unsure how this occurred.
