Thanks for that Iguana - good photo! Could be a couple of things, more likely related to physical damage - either a burn, or if you were handling at it a critical time before it shed, the old layer of skin may have been torn away from the developing skin underneath, possibly if you were unwinding or dragging it from a branch or whatever. So, check for possible hot spots where the animal may burn from a heat source - this is more common where a heat source gets hot very quickly when the thermostat turns it on and the snake is close to it, leading to intense localised heating before the animal warms up enough to move away. The other thing - handling when in shed - is self evident. Once you notice the shed approaching, don't handle it at all, just leave it be until the process is complete.
Jamie