Not OP, but worked in the field for a time. Trafficking usually means configuring your ad in the campaign management system. What are your targeting parameters, what are your tracking tags, uploading the ad itself, entering in lots of custom information that while conceptually similar across ad platforms usually has different names and often has to be manually entered. A lot of platforms do offer APIs of some sort to help with bulk campaign/ad creation, but there's often no "one stop shop" to be able to set up a google campaign and facebook campaign at the same time. There are some companies working on this, I think usually referred to as (or in conjunction with) "marketing automation".

A mistake can mean - misconfiguring your target (wasting money on ads that won't give you an ROI), misconfiguring your 3rd party tracking (letting data like conversions go unaccounted for, or not having your auditing tags setup, meaning you show ads to fraudulent users that you otherwise wouldn't have to pay for), etc.