This just happened to me, and I hadn't initially realized that it was the separation of the splined shaft from the head piece. Confused I was when I tore down the unit to replace both the pin and the lock ring in the upper shaft which weren't broken in the first place, but of course after I removed them, the unit fell out just like it appeared to do during operation. Once I re-assembled and it still occurred that is when I decided to pull the separating part completely off and saw the spline, and then inferred that since it only went back in one way, that it was stripped enough to loosen. I sold off all my shop equipment, so I think I am just going to buy a replacement part. Cheaper than a new mixer. I use my mixer every day, with attachments, so I feel like that unless I could drill it out and insert a pin or set screw, it's only a matter of time before something like increasing the friction tightness would fail again, because bread dough. That is probably what loosened it in the first place.