A special cause producing temporary deviation in the underlying process can influence on process adjustment in First-Order System feedback control system. In this paper, the impact of special causes on the forecasts and the process adjustment that is based on the EWMA forecasts are derived for a first-order system. For some special causes with patterned type of contamination, the influence of the causes on the output process are explicitly investigated. A data set, contaminated by a special cause of level shift, is analyzed to confirm the impact numerically.