Here my top 10
1-UVB Lightening (old, wrong, too close or too far UVB source.. 6 month later boom.. MBD)
2-Water : Stupid waterfall.. or dish.. this is useless and lead to dehydratation or disease (bacterial disease caused by the cultire inside the water source)
3-Temperature (What temperature needed (1-what is the chameleon speacies.. veiled not equal too jackson)? He need heat source to digest food, activate metabolism and immunitary system) soo.. 6-8 month later boom

MBD (what?! mbd again?! yeah.. if he can't digest the food how he gonna absorb the nutriment?

) he also need colder spot without uvb (too much UVB like.. not enough uvb
4-Burn : i have made this mistake.. i lost one of my chameleon because i was sure he can not reach the lamp.. that was a mistake.. he reach it and burn the tail from A to Z.. he died 4 month after.. alway put your heat lamp in a safe place and but heat source for tropicalspeacies and please test it before (is the source are too far?! too close? with infrared gun (about 14$.. no big deal to safe your chameleon life)
5-Over/under suplementing : too much = toxicity no enough =.. not enough

both lead to severe and long terme dammage
6-Handling the chameleon ("but he love me!" well.. yes/no.. he dont love you he tolerate you.. major of the time if he REALLY want to go out the enclosure its because outside better than inside.. if the enclosure is wrong he will try to escape the enclosure and if you open the cage he will grap the opportunity.. Handling too much your chameleon is stressfull, wrong enclosure is also stressfull and it can shut down the immunitary system (also.. if you manipulate your chameleon without washing your hands.. boom.. respiratory infection (but why?! : juste because human can carry bacteria and the chameleon donc have defense (unlike human) againts this bacteria (its like inverse model of salmonela

)
7-Housing chameleon together : "he need à girlfriend" well.. come on.. he dont need a social life in a small enclosure.. figure yourself 24/24 7/7 with the same person.. and with the same guy who always just too have sex or raping you major part of the time.. lead to stress or fighting
8-Superworms = #1 feeder : this is a mistake because ratio between calcium/phosphorus are not amazing.. and, variety = best #1 feeder
9-Glass enclosure : this is not critical.. and can be ok with experienced owner but still a mistake with beginer. major reason : there is no air flow, can lead to bacteria. Second reason : can lead too stress because glass act like a mirror (also.. classical exo-terra 18x18x24 are just too small.. btw small enclosure also not good!
10- To be stubborn.. when someting wrong.. put your ego in a small box.. this is useless

ask, found a solution, read and accept if you have make any mistake!

(hey.. i think in the 10 mistake here.. i make about 5 of them in the past no reason to be stupid and not fixing the problem before its too late!)