Media planning used to be a discipline learned over a long period of time. It was taught at the big agencies with manuals constructed from mimeographs collected over years, passed down from media director or associate media director to planner, and then passed from that planner -- once he or she became an AMD -- to the next generation of planner.

The authority of media planning was rigorous and exacting, carried out with monastic dedication, weighted with a sense of certainty and rectitude that was something between science and religion.

Today, media planning, particularly as it is applied to online, is practiced with more of a sense of certainty borne of support by research and technology, and less a sense of rectitude. Plans are put together in a frenzy, @Plan or comScore runs sorted sites loaded into the ad server's media planning console, and negotiations are handled impersonally, virtually, and post haste. Technology rules the day. And because it seems to make everything move faster online -- and it actually does -- everything is subject to time compression, and clients regularly require short turn-arounds on all their planning. It means that the modern online media planner is exposed to the kind of pressures that would make a diamond blush.

It also means that mistakes get made. I don't mean the kinds of mistakes that find their way into a spreadsheet, such as miswritten formulas, adding a column and missing a cell, misspelling a client's name. These are all common -- though hopefully infrequent -- mistakes that typically get caught in the process of the media buy's manufacture, but sometimes aren't caught until the presentation to the client.

Rather, I'm talking about mistakes that are more fundamental -- mistakes that are there from the start, sewn into the lineaments of a media plan. I'm talking about mistakes that are the result of a changing definition of intelligence and a fading respect for deliberation, and a growing admiration for sudden decision-making. These are mistakes that doom a plan from the start, or at least render it sterile and ineffectual, or, if not that, make it a complete waste of time and money.

Seeing as how time and money are the two most valuable assets a business has, these types of mistakes are the ones that matter most. Following are six such blunders that media planners make during the planning process. These can work their way into the fabric of a media plan and mar the outcome of the whole tapestry.