I have access to an ArcGIS Online account. I wanted to create a map with some polygons. So I've used QGIS to make the shapes, saved the layers, zipped them, and uploaded them to ArcGIS Online.
Two days ago my polygons uploaded great and aligned to the basemap perfectly. Using WGS 84 / Pseudo Mercator EPSG:3857 in QGIS worked like a charm (You can see how the shapes border the river very well).
Today I made some revisions to my shapes, including joining some data tables and moving some nodes around. Now when I upload the shapefiles (with the same CRS), everything is misaligned about a mile South of the river.
In QGIS, these two layers overlay as expected. So, what happened?
Here's the results of my .prj file (broken into lines for readability). I have to admit, I have no idea what I'm looking at:
PROJCS["WGS_84_Pseudo_Mercator",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984", SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0], UNIT["Degree",0.017453292519943295]],PROJECTION["Mercator"], PARAMETER["central_meridian",0],PARAMETER["false_easting",0], PARAMETER["false_northing",0],UNIT["Meter",1], PARAMETER["standard_parallel_1",0.0]]