#1: Over Henna'ing
I notice that most people space out their henna applications by at least 2 weeks. I have however been doing them weekly for the last 3 weeks. I didn't notice any color change until recently (I had dark brown semi perm color prior to henna). My hair is naturally a light dusty brown color and is almost orange now. Not sure how that happened on the dark brown but now I am a little worried about my hair after using henna that often.