A penalty in which either team has too many people at a particular moment in play. On offense, a team can be penalized for illegal substitution if they have twelve players in the huddle at any given time between the formation of the huddle and when it breaks. On defense, illegal substitution can be called if a team has twelve players on the field at the snap of the ball, even if one of those players is attempting to leave the field of play. Illegal substitution is a five yard penalty and the down is replaying. The offense has the option to take the result of the play rather than the five yard penalty.
Most illegal substitution penalties occur when there is some confusion over who is supposed to leave the field of play during a change of personnel or formation. On offense, a player may not leave the huddle because they are unaware that they are being substituted for. The quarterback can usually help with this by standing away from the huddle until substitutions are completed. On defense, a player may be unaware that there is a personnel change and forget to sub out for a different player. Too many men on the field is the most common illegal substitution penalty.