Mr. McGuire says his horse, Mercury, "loves buttercups". Buttercups are poisonous and have a acrid flavor. A horse likely wouldn't eat them and if it did, would become quite sick.
When Mr. Wilson goes outside to try to buy a Christmas tree from Mr. Maguire, the shadow of the boom mic crosses over the trees in Maguire's wagon - even though they are in the middle of the street and there would be nothing to create such a shadow naturally.