Revealing mistake: When Eric picks up Cory to show him the "For He's A Jolly Good Fellow" dance, Cory screams that Eric is going to knock over the candles, upon which one of the candles on the desk is knocked over and sets fire to the curtains. If you look closely, neither Eric or Cory bump any of the candles nor the desk they are on, the candle just tips over on its own, suggesting an off-screen crew member used wire or string to pull the candle down. (00:07:18)
Revealing mistake: When the family is eating dinner, throughout the scene you can see Jack dabbing his fork into his mashed potatoes and raising the fork to his mouth with little to no potatoes on it in order to give the illusion that he is eating. (00:03:57 - 00:05:05)
Revealing mistake: When Cory puts the talking teddy bear that Topanga gave him in the trash compactor, the bear screams and Alan and Amy come down to investigate. Amy opens the trash compactor door and pulls out a bear crushed into a cube, but if you look closely, you can see the original bear still in the trash compactor unharmed.
Revealing mistake: During the montage of Eric and Frankie's game of pool, at one point Frankie hits two balls that spin towards a pocket in unison. When both balls come to a stop, you can see a dent in the green ball where a wire or other device would have been inserted to keep the balls linked together.
Revealing mistake: When Amy steps into the living room and sees Reginald, you can see a plain white wall behind the door that leads into the kitchen, when you should be able to see the kitchen itself, which would suggest another set was placed there and they didn't move it for the filming of this scene.
Revealing mistake: When Cory and Shawn are holding band auditions, if you watch the guy that is shredding on guitar, you can see that although he is moving his fingers all over the fret board, his picking hand remains motionless, showing that he isn't actually playing and the sound was added in post production.
Revealing mistake: When Eric tells Cory and Shawn that the hot chocolates were $4, Cory looks up at the prices and says "that's the retail price." As the scene and the rest of the episode go on, if you pay close attention to the sign that has all the prices, hot chocolate is not an item.