A tenth of a millimetre sounds like a purchasing detail. On a large flat panel it changes the part. Bending stiffness rises with the cube of thickness, so moving from 0.5 mm to 0.6 mm makes the panel roughly 73 per cent stiffer while adding 20 per cent more steel. That is useful if you set out to buy a stiffer panel. It is a problem if the change happens quietly, after the drawing was approved, because everything around the panel was built for the original number. The flange stack gets thicker, so a fastener that was correct now sits short or bites too deep. The panel sits further out in its frame, which shows up as a gap line that no longer matches the door beside it. Springback changes, so a bend angle that used to land on nominal now drifts, and flatness on a thin wide panel is the first thing to move. If the part is welded or riveted to a mating component, the stack-up error is transferred to that component too. On the line, the difference is harder to see and more expensive to fix: the parts still fit, they just do not fit the way they did last year. The practical rule is to fix thickness at the drawing stage, tie it to the stiffness requirement rather than to the steel price that week, and treat any substitution as an engineering change — quoted, sampled and approved, not swapped in by the purchasing department.