In "Turncoat Smurf," Handy told Hefty that he and the other Smurfs searched everywhere for Brainy who escaped from the storehouse where he was imprisoned for allegedly helping Gargamel. In a following scene, the Smurfs (through Clumsy's off-screen exclamation) learned by checking on the storehouse that Brainy escaped even though they already knew this in a previous scene.
In "The Sky Is Smurfing, the Sky Is Smurfing", when Hefty and Scaredy are flying on Feathers, upon landing, Hefty's usual red heart tattoo is pink.
In "Turncoat Smurf", when Smurfette and Hefty realize that Brainy has escaped from the locked room, Clumsy can be heard saying, "Oh, gee! Gosh! Brainy's gone!", but he is nowhere to be seen.
In "Turncoat Smurf", when Brainy is waking up Papa Smurf, he and Hefty are the only ones in his lab, but when Gargamel is in the Smurf Village, Smurfette appears out of nowhere in Papa Smurf's house.
In "Turncoat Smurf" when Papa Smurf remembers about the personality exchanger formula when Gargamel is attacking the Smurf Village, the bottle appears on a table in what looks like Gargamel's hovel instead of Papa Smurf's laboratory.
In "Turncoat Smurf," Gargamel used a bulldog as a "fool-proof" way to catch Smurfs. A time latter, he used the bulldog to track the scent of Brainy's clothes to locate the Smurf Village. However, bulldogs were not bred for their acuteness of smell during the Middle Ages (the background of the Smurfs cartoon series) but bred to bait bulls.
When Brainy is writing at the end of "Turncoat Smurf", his arm disappears and reappears.
In "Turncoat Smurf", when Gargamel is dumping Brainy's clothes out of the bag, you can see there are some shirts, despite the Smurfs never wearing any shirts.
When in "Turncoat Smurf", Hefty and Smurfette are locking Brainy up, Smurfette says, "We're going to keep you here until Papa Smurf wakes up!", her mouth does not move.