Illegal procedure is a penalty in football that refers to a wide variety of infractions against the offense. It is always a five-yard penalty.
Many different types of infractions can fall into the category of "illegal procedure", including illegal shift, illegal motion, illegal forward pass, illegal touching of a forward pass, ineligible receiver downfield, illegal substitution, false start, illegal formation, kickoff or safety kick out of bounds, or a player voluntarily going out of bounds and returning to the field of play on a punt. These infractions are not always called "illegal procedure" by the referees, as this is too vague of an explanation. The referee will usually call it "illegal procedure", and then go into greater detail about what the illegal procedure was.