When Kate brings the packages to the post office to be mailed, none of them have any addresses (or anything at all) written on them.
On Christmas morning Kate tells everybody that dinner is at 1 o' clock and then says she has to go out and get a turkey. Turkeys are frozen at the store and need to defrost at least 24 hours before they can be baked. There is no way she would have dinner ready in time.
Holly uses her bare hands to knead green food coloring into cookie dough, yet her hands are not green afterward.
When arriving at the theater, Kate is being repeatedly shushed by the audience while she's yelling at Chip, and she finally shushes the audience right back; at that moment an extra right behind her--a blonde lady wearing a pearl necklace--can't help herself but grin broadly and sweetly at Melissa, thereby breaking character: she, (along with the rest of the audience) is supposed to be very upset at Kate for yelling so much and disrupting the peace, but we can tell from that lady's sweet smile and tender look at the actress, that she was amused by how well Melissa performed her aggravation in that scene. This goof turned into a very cute moment.