In "Scruple and the Great Book of Spells," the Book of Spells flips to a page showing pictures of a tomato plant, a corn plant, and a potato plant. However, tomatoes, corn, and potatoes were unknown in Europe until they were imported from the Americas in the 16th century. The background of the Smurf cartoon series is the late Middle Ages (circa 14th-15th century), early for the understanding of tomatoes, corn, and potatoes in Europe.
In "Scruple and the Great Book of Spells," Farmer's crops were affected by the depletion of iron in the soil due to Scruple creating an iron statue of himself near Gargamel's hovel. However, other plants in the forest were not affected by the iron depletion, especially those who are nearby Farmer's crops.
In "Bouncing Smurfs," Papa Smurf by examining the rubber sap he collected said that its magical properties affect living things which became evident when Clumsy accidentally fell into the sap tub and became bouncy. However, Papa Smurf's right hand was not affected even though he had direct contact with the rubber sap (drops from it fell from his hand) when he scooped a sample for testing before learning about its properties. His right hand should be rubbery, but it did not.
In "Scruple and the Great Book of Spells," the Book of Spells told Gargamel to gather the following ingredients he need for a spell to catch Smurfs: henbane, mudwort, fresh mildew, pond scum, marsh moss, and stinkweek. However, when the Book of Spells flips to the page picturing the ingredients, it showed pictures of a tomato plant, a corn plant, and a potato plant. These are not the ingredients the Book of Spells told Gargamel to collect.