Howdy Tara,
I'm in the "camp" with those who suggested an overdose of vitamin A. Injectable vitamin A is water-soluble. This form of vitamin A has a much higher chance for overdosing compared to the fat-soluble form such as the liquid gel-cap type that many of us have used.
From Dr. Mader's "Reptile Medicine and Surgery" 2nd Ed. (Most of this was aimed at turtles but as you can see, it hits the mark for your chameleon

.)
"Treatment of hypervitaminosis A is essentially identical to that of severe burns. Prognosis is dependent on many factors, the most important of which include the dosage of vitamin A, the health status before administration, and the aggressiveness of treatment….”
“Until more is known, avoidance of water-soluble vitamin A and use of fat-soluble vitamin A instead seems prudent.”
“…early retinol toxicity primarily affects the epidermis, initially causing dry flakey skin (xeroderma). Within a few days, the erythematous areas spread over the skin.”
“.”The blister, or bullae, break open and expose moist, reddened, or gray underlying tissue and eventually slough.”
“Water-soluble vitamin A produces these lesions within 10 to 15 days, yet fat-soluble vitamin A at identical doses did not, even though liver levels of vitamin A were similar.”
“Further confusing the entire issue is the fact that elevated intakes of vitamins D, E, or K may reduce vitamin A assimilation or restoring their respective adequacies. Protein deficiency may have a protective effect against hypervitaminosis A.”
EDIT: (From page 1069):
"These results usually happen at dosages of 10,000 IU/kg or higher given IM (intramuscularly) as a single injection. Treatment involves ceasing vitamin A administration, antibiotics, fluid therapy and nutritional support. The skin lesions may heal slowly, but animals managed supportively can completely recover."