Mark unknowingly eats a walnut and has a severe allergic reaction immediately. This was pointed out as a goof because he ate almonds with no problem in an earlier episode; however, an allergy to one type of nut does not mean that someone is allergic to all or any other nuts.
Mark Saint-James eats a walnut from a muffin Betty gives him, and it is discovered that he is allergic to nuts. However, in the previous series, during the budget cuts at Mode, Wilhelmina is forced to swap her afternoon caviare and champaign for almonds and soda. Mark eats an almond in front of her, and has no problems with it whatsoever.