Not knowing the implications of the PID Form in an existing control system being migrated or the PID Form learned in a University course can cause gross errors in the tuning parameters and potential instability. The PID Form predominantly used today is not the Form in most of the controllers installed before the 1990s and is not the Form seen in most control theory textbooks. Here we alert you to the differences and consequences. An Appendix provides simple equations to convert between the different PID Forms and block diagrams in the time frame that are not available in the literature.