Listening too closely to manufacturer hype, like megapixel cameras can be used as tag cameras at night.
Saying too often "we can do that" or "will do that" without knowing for sure if we can or will.
Failure to manage customer expectations, like the low reliability of recording cameras across an Internet connection means very low framerates and occasional dropouts.
Talking too much and interrupting the customer when they are trying to tell you their needs.
Crossing the line of being overly focused on price alone like a retail seller and not communicating the value of the services being offered as an installer and support company.
Chesapeake & Midlantic
Believing manufacturer propoganda. Simply parroting manufacturer claims without investigating their accuracy, or even considering the likelyhood of that accuracy.
Being overly price focused. Being afraid to tell the customer "no, your budget isn't nearly high enough to buy what you'd need to accomplish your goals. Either adjust your budget or adjust your expectations".
Overpromising on timelines- being overly optimistic on how long tasks will take.
Airing dirty laundry in front of the customer. Complaining about management or installation staff in the customer's hearing, or overly agreeing with the customer's complaints. This is a major rookie mistake.
Not knowing how to close. Taking too long to close.
Allowing the customer too many free change orders. You let two or three go for free as a courtesy, and the customer makes a thousand change orders and the job simply never gets finished because the customer is never satisfied.
I'm sure there are others.