The worst mistake I've come across is when they claim to know your curls better then you do. (I've had this hair for nearly 50 years. I think I know what it does.)
I told my previous hair lady that my curls are tighter and thus shrink up more on top and front (4A) than at the nape (more 3C). So I asked her to cut more in the back.
She said, "No. Your hair is curlier in the back." So she didn't cut much in the back as I asked. The cut initially looked good... for a few weeks. Then the "curly mullet" was beginning to form.
Yeah... I don't think she'll get near my hair ever again!