I work in digital marketing and have worked in a couple marketing departments handling fairly large levels of digital ad spend ($1M-$2M/month range). If you're not careful with that level of spend, small mistakes become big mistakes very quickly - this wasn't my mistake but a close colleague's.

He set up a large digital ad campaign which was going to spend around $100K over the course of 3 days, easy enough - the problem was, he set-up the entire campaign without recognizing a vital 'ad objective' error, that would decide which action the ad server would look to most often complete with the $100K budget (think email sign-ups, views on a promotional video, or ecomm purchases)

His error resulted in $100K of client ad dollars down the drain, as the ad server spent the money with no intent on achieving our client's end goal, which ultimately produced results that the client couldn't have given less of a shit about and saw as a huge waste of money. Our beloved colleague was fired soon after to save face in the eyes of the client.