Printed this cylinder as part of a larger project and there's a horizontal line that runs up most of the part. Not bad enough to scrap the project but I'm curious if anyone can explain what causes this. Using an Ender 3 printing PLA, Cura slicer.
This is quiet normal and not an error. It's the Z-Seam, caused by the printer when finishing a layer and briefly stopping the printhead, before lifting it along the Z-Axis to the next layer.
Here you can find more about this effect and how to reduce it.
Pick random for the z seam setting and it'll put it all over the print. This sometimes causes a worse effect, but if your printer is well calibrated then it'll be less obvious. I think there's even a setting to randomize it near the back of the print or near corners. The latter probably won't help much if you're printing a cylinder.