The lights flickered, the printer restarted, and it offered to continue. It did continue — and everything above the resume point is offset sideways from everything below it.
Recovery restores a memory, not a position
When power fails, firmware with loss recovery writes its last known coordinates to storage. On restart it reads them back and carries on from there.
The gap in that reasoning is what happens in between. With the motors unpowered, the toolhead is free: a moving gantry coasts, a belt under tension pulls the head a little, gravity moves a bed-slinger's plate, and anybody who nudges the machine while investigating the outage moves it further. None of that is recorded, so the machine resumes from coordinates that were true at the instant power was lost and are not true now.
Firmware that re-homes X and Y before resuming avoids most of this, because it re-establishes the origin from physical stops rather than trusting memory. Firmware that resumes without homing cannot.
The other damage, which is not an offset
Even a perfectly positioned resume has a second problem. During the outage the part cooled, sometimes for hours. The layer printed onto it lands on cold plastic, so the weld at that interface is far weaker than the ones around it.
The result is a part with a designed-in fracture plane at the resume height. For a decorative print that may not matter. For anything structural it matters a great deal, and it is worth knowing before trusting a resumed part with a load — this is the same mechanism described on weak layer adhesion, concentrated at a single plane.
A part that also detached slightly during the cool-down compounds it, since the resumed layers are then printed onto something that has moved.
Deciding whether to resume
Resume if the print is decorative, the outage was brief, the part is still firmly attached, and the machine homes before continuing.
Start again if the part carries load, the outage was long enough for the part to reach room temperature, the machine does not home on resume, or the part shows any sign of having lifted.
Treat recovery as a salvage attempt with a decent success rate rather than a guarantee. It is a genuinely useful feature and it is not a substitute for stable power.
Preventing it
- Do not touch the toolhead during an outage. Not to look, not to move it out of the way, not to remove the filament. This is the single most common way a recoverable print becomes an unrecoverable one.
- Fit an uninterruptible supply if outages are common where you are. A printer's steady draw is modest — a small office-grade unit will carry a machine through the brief dips that cause most of these events, and brief dips are the majority.
- Check the supply, not just the grid. Repeated unexplained restarts on a machine nobody else complains about are often a failing power supply or a loose mains connector rather than the mains itself, and that is a fault worth finding for its own sake.
- Look at what else is on the circuit. A heavy load starting up — a workshop tool, a kettle, a compressor — can drop the voltage enough to reset a printer's controller without ever disturbing the lights.
Brownouts are worse than blackouts
A clean power cut stops everything at once. A voltage sag can leave the controller running while the motor drivers lose their supply, so steps are commanded and not executed and nothing detects it. The machine continues printing, apparently normally, from the wrong position.
That is why a shift can appear after a flicker that did not restart the printer at all — the recovery system was never involved, because from the controller's point of view nothing happened.
Distinguishing it from an ordinary shift
If there was no interruption, this is not your page: layer shifting covers collisions, belts and drivers, and it is far more common. The tells for the power version are that the shift coincides with a known event, that it often affects both axes at once rather than one, and that the print resumed from a stop rather than continuing without interruption.
Material plays no part in this failure, though a high-contraction material makes the cool-down more damaging: an ABS part left unheated for an hour has contracted away from the plate as well as gone cold, so both problems arrive together.