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I recently retrofitted the E3D Ver6 hot end, and have so far been really pleased. I made 2 parts with it, then started part #3. It was building normally, left it alone for about an hour, came back to find it making a rats nest. It was printing a repeated pattern, well above and outside of the part. Just a back and forth motion, over and over. The layers that showed on the computer screen were normal. Any thoughts on what might have caused this?

Loose belts come to mind, or maybe it hit a blob and displaced the effector platform.

I'm going to guess that there was a bump on the top surface, possibly from over extrusion, or a section started curling up, and the machine pulled it loose from the build plate, and also forced the hotend to skip steps and end up out of position, from having that exact thing happen before.