The morning after the big storm, as Jim walks from the cab to the house, everything is perfectly dry, even though it was pouring rain the night before.
When driving out to the house for the first time, Mrs. Blandings' corsage is placed very high on her lapel, nearly on her shoulder. While she's getting out of the car and taking the initial look at the surroundings, it is placed several inches lower.
Jim was taking Muriel a tea and the tea cup and saucer were in his left hand, when he entered the bedroom the saucer and tea cup were in his right hand.
The foreman asks Mr. Blandings if the "lintels between the lally columns" should be rabbeted. However, the lintels are on the second floor of a two-story house. Lally columns are used in the basement or the first floor to provide support to the middle of long beams. They are not needed on the second floor.
Early on, Mr. Smith advises the Blandings that a trout stream running by the old Hackett place will furnish them with their water. Later, much ado is made about drilling a well. This discrepancy is explained the night after the family moves in, when Mrs. Blandings complains about a "dried-up trout stream".
On the architect's first blueprints, he points out three bedrooms, two of which have an adjoining bathroom. Only one has such a bathroom.
When Betsy cuts the classified ad from the paper, she says "It's just sixteen words" then proceeds to read "Forced to sell farm dwelling original beams barn apple orchard trout stream seclusion superb view will sacrifice". Seventeen words.
The morning after the big storm, Mr. Blandings says that he hates the house's rabbeted lintels. However, the carpenters had already removed the rabbeted lintels in an earlier scene, after Mr. Blandings said that he didn't want the lintels to be rabetted.
(at around 46 mins) In a drawing of the new house in the architect's office, "Blandings" is incorrectly shown with an apostrophe between the "g" and the "s".
Jim Blandings drives a 1941 Buick convertible. This is inconsistent. A well paid NYC account executive would at least be driving a post-WWII model automobile (1946-1948) by this time.