Thanks for the reply! That seems like a good place to start looking. I am still new to arcpy/GIS in general and don't know much about working with spatial references.
I'm reading coordinates from one GIS(ish) system that uses Web Mercator (EPSG:3857), using those coordinate pairs to create points (arcpy.Point(latitude, longitude)), creating an array of those Points, then using that array to create a Polyline (arcpy.Polyline), and then using arcpy.da.InsertCursor to insert the polyline into the ArcGIS database. I read somewhere that in order to do high precision geometry, the polyline needs a spatial reference, so I've been using a blank one:
poly = arcpy.Polyline(point_array, arcpy.SpatialReference())
which has worked every time so far.
The ArcGIS feature class uses GCS_WGS_1984. Should I be specifying anywhere that I am using coordinates from 3857? How should I go about doing this?
Thanks for your help!