Quick answer
A coiled extension lead carries the same current as one run out flat, but it cannot shed the heat. Wound tightly on its drum, every turn warms the turns beside it, and the temperature in the middle of the coil climbs well above the rating printed for the cable in free air. Under a heavy load, a heater or a welder for example, the insulation can soften or even melt while the wall socket still reads normal. The safe practice is to unwind the lead fully before drawing a large current through it, and to check the drum's rating, which is often quoted separately for the coiled and the uncoiled condition. APAR light-duty and flexible cables carry their rating on the sheath so the limit is always to hand.