I would say one of the biggest (especially if your looking to combat frizz), is when they tell you their helping you out by thinning your hair out and they whip out a trusty RAZOR
. That is a no no because it will have individual, uneven strands that will most likely resemble frizzies.
Another one would be when they cut your hair by straightening it to make sure they are cutting "evenly". This is a problem especially if your one of the "I only straighten my hair when I go in for my cut types". It just doesn't make sense if you leave your hair curly otherwise.
Finally, when they spray it wet and cut it saying they will take an inch or less off. They end up cutting more like 3-6 inches in reality and then you deal with lovely shrinkage.
I have a phobia of hairdressers.