There is something faintly ridiculous about a machine that fixes itself, and the cheap printer in the corner has spent the holidays doing exactly that. A filament guide snapped. So I printed a new filament guide. The spool holder bearing was rough, so I printed a press-fit replacement. The cable from the hotend kept catching, so I printed a little clip to route it. Every one of these came off the very bed they were repairing.
It feels like cheating, and it is the best argument for owning the thing. A commercial appliance that breaks a small plastic part leaves you waiting two weeks for a moulded replacement, or it leaves you binning the whole unit. Here the broken part is a fifteen-minute reprint in PETG and a download, or twenty minutes in a CAD package if nobody on Thingiverse has done it first.
The honest caveat: you can only bootstrap so far. The printed parts are the brackets, guides, fan ducts and clips. The bits that actually have to be precise or hot or under real load, the rails, the bearings, the heater, the board, still come in a box from somewhere. So it's not quite a machine that builds itself. It's a machine that maintains itself, which is the more useful trick anyway.
The clip is holding. The guide is holding. The bearing is quieter than the original. Next year's first job is already printing: a slightly better version of the part that broke, so it doesn't break the same way again.