Case length and deformation of case mouth.
If a fault is detected the tool head moves to top position and then stops the operation. Faulty case is removed manually. Powder drop and bullet feed is automatically turned off for this case position.
Machine stops, remove faulty case and press start. Simple.
No manual repositioning of tool head. No need to put a new case in too replace the faulty one. No risk of powder spill or a bullet falling into the empty position.