The movie is notable as the third theatrically released film to feature the character of Ray Stantz as portrayed by Dan Aykroyd. Stantz is called in by the film's villains to rid a haunted house of its previous ghostly owners, but he is unable to do so saying, "Who ya gonna call? Somebody else."
Aykroyd sports a mustache for this appearance. The possbile explantion for this being some actors performing in cameos see it not reason enough to alter themselves for a short scene. Especially if the look is for another role they are currently devoted to.