What is one of the worst mistakes you can make during salary negotiations?

Leaving money on the table. Early in my career, when I was young and very naive, I did not know that usually a company's first offer is at or below the midpoint of the salary range for that position. I naively assumed that a company's initial compensation offer was (a) the best they could do, and (b) not negotiable. Remember, your salary growth over your career will have a start point. Wouldn't you want that start point as high as possible?

Another thing to keep in mind. The company EXPECTS you to negotiate a better offer for yourself. If you don't even attempt to negotiate, this can send very negative vibes to the company. These negative vibes might include one or more of the following: (a) you're desperate and will take any offer , (b) you lack drive and initiative, or (c) by not even attempting a counter-offer, you show that you lack leadership skills and are definitely NOT management material. This could contribute to the company's perception of you as "deadwood" and you are more likely to get laid off.