We're using ArcGIS 10.5.1 for an undergraduate class in GIS. In that class four separate people had an issue in adding XY data. In all of these instances, it would plot somewhere wrongly. However, if they went back to the original spreadsheet and changed the column headers from LONG/LAT to X/Y it plotted correctly. It wasn't a simple transposition error of them getting the Xs and Ys the wrong way round as I doubled checked each of them fully.
This seems really odd, and I'd be interested if anyone else has had anything similar. I've had nothing but trouble since our most recent upgrade, so am really considering rolling back versions as it doesn't make for a great learning experience at the moment.