When the nephews sit down after the "play" you can already see feathers on the back of the chair indicating it is at least the second take.
At 4:30, Harriet Oleson calls Almanzo "Zaldano". This has happened in many episodes before and after this one. She can never get his name right.
The boys ran straight through Laura's vegetable garden. No prairie pioneer household would grow vegetables without have a fence to keep out rabbits, deer and other critters.
At 4:25 when Almanzo is in the post office with Melinda Foster as the post woman, the back of her head is shown and you can tell her bun she has her hair in is fake. Her bun is a reddish color while the rest of her hair is brown.
While Amonzo and Laura are busy taking care of their nephews, the parents go on a rafting trip on a white-water river somewhere. They are seen repeatedly lying on a raft, in bathing costumes, ostensibly steering the raft with a rudder. Without some sort of propulsion, a raft cannot be steered with a rudder; it would only rotate instead of move forward relative to the water. Also, it appears that the water is moving quickly relative to the raft: again something that would not happen.
When Laura, the nephews and him are by the lake, Almanzo (in particular) is wearing contemporary clothes that decidedly give him a late 70s/early 80s silhouette: corduroy pants with a tight-fitting waist in a straight, latest fashion cut, as well as a brand new & modern leather belt. Earlier, a man was also wearing a modern turtleneck with a chunky collar that stood up nicely and straight, the kind that were definitely not in existence in the era that this show is supposed to represent.
While an exact date for this episode is not apparent, it appears to be occurring in the late 19th century. Early in this episode, Royal Wilder offers the "nephews" an Indian nickel if they behave well on their vacation. They then proceed to bargain for a Silver Dollar to split between them. The "Indian nickel" (aka Buffalo nickel) was not minted until 1913 and the nickels of that era were Liberty nickels with a few older Shield nickels last minted in 1882 still floating around in circulation. There were also Indian pennies in circulation at that time. In any event, the coinage doesn't match the time period.
Almanzo is seen wearing pants with belt loops which weren't invented until 1922.
In season 8 Royal Wilder has a wife, 2 boys and another on the way. Season 9 Royal returns . With no wife or boys and only the daughter Jenny who is about 9 or 10 years old. The time frame between season 8 and 9 are roughly about a year.