Bleed.

The strip of artwork that runs past the trim line and is cut off after printing, there so that a cut slightly off position lands in image rather than in white paper. Gelato requires 4 mm on all sides of a print-ready PDF, and Prodigi says photo prints typically have 3 mm. Not every product wants the same strip: on canvas the band that wraps the bars is a separate and much larger allowance, worked in the stretcher bar entry. Gelato prints most drinkware exactly as supplied but requires 2 mm top and bottom on an 11 oz ceramic mug, and on Printify's all-over products the bleed goes into the inner seams instead of the bin. A DTG shirt has no trim and no bleed at all. The common failure is a file built to the exact product size and sent to a lab that cuts, which Gelato flags because it leaves unwanted white borders on the finished piece. A white hairline down one edge is enough for a return on wall art, and the reprint costs the paper, the ink and a second shipping label.