Faulty predictions based on maintenance of the status quo
Standard "business as usual" scenarios do not take into account the dynamics of past change - the past rates of change - but rather use the present baseline as the measure which is projected into the future. They also generally fail to account adequately for humanity's capacity to adjust to change. These assumptions produces faulty outcomes when there has been a significantly increasing or declining rate of change in the trend over the historical period, or where the potential for behavioural adjustment is high.
Population projections have been continually pulled downward in the past twenty years from the overinflated projections based on birth rates whose upward trend was projected forward, ignoring the more recent small, but steady, downward rate of change. Similarly, predictions of energy use, resource depletion, extinction of species, and other such statistics have all been strongly adjusted up or down in recent years as a result of previously ignored or undetected rates of change in the observed trends, or unforeseen changes in individual behaviour.
Long-range planners should spend more time thinking about the evolutionary opportunities and implications of development trends, rather than assuming continued development of well-established technologies, infrastructures and ways of living.