The NFL is very specific about what each player must and must not wear when on the field. There are many pieces of illegal equipment that players are absolutely forbidden from wearing. This is an infraction, and the player may not play on a given down if they have any illegal equipment on their person. The player may return on the next down once the offending articles are removed.
Some examples of illegal equipment include: projecting objects that may protect a player, hard objects and substances, including casts, guards or braces that are uncovered, torn items, improper cleats, items colored like a football, prohibited head wear, prohibited glove colors, and many other items. Each and every uniform item is listed specifically in the official NFL rules, and players are expected to know and follow them. Players who consistently disobey uniform and equipment rules may find themselves being fined by the League office.