#9 Not knowing signal flow in the mixer
Not knowing the signal flow in a mixer will be a source of frustration for the Sound person. "Back in the day," a sound person in training had a difficult enough time navigating the rows of knobs on analog consoles. Now that digital desks have taken over, you have to navigate sometimes many layers of menus. Some consoles are more intuitive than others. Some are just plain hard to get around in. There was always "more than one way to skin a cat" in the analog realm. It seems with digital there are hundreds of more ways. The best way for me to learn digital mixer shortcuts, layers and tricks are manufacturer provided classes. Another great resource is YouTube, and then other FOH engineers have great ideas. There are a lot of resources available. Know your console and your particular console setup. You can't mix if you can't get around your console!