Continuity mistake: After the brawl between the villagers and the objects, a lot of mess is on the floor. Then when the objects jump for joy, the mess is gone.
Continuity mistake: When Maurice gets his woodcutter working in the cellar, the axe starts cutting a log which is in a wooden bench, which sits in front of the woodcutter. However when Belle entered the cellar earlier, through the clouds of dust, the area in front of the woodcutter was empty. (00:08:45 - 00:10:05)
Continuity mistake: In the shot of Belle reading her book in her house, the spout of the black kettle on one of the shelves is not visible. In the following scene, when Gaston is talking to Belle with his feet on the table, the spout is visible.
Continuity mistake: When Gaston is cleaning his teeth in front of the mirror, the decorations on the mirror's frame change between shots.
Continuity mistake: When Maurices invention explodes, Belle runs to the house and into the cellar. As she enters, you can see the area in front and around the stove/fireplace is empty. However after talking for a while, Belle says "...oh Papa, he's not for me" and sits down on a wooden stool which has suddenly appeared in front of the stove/fireplace. (00:08:45 - 00:09:45)
Continuity mistake: When Gaston falls out of Belle's house when she opens the door suddenly, Gaston lands in the pig pen. He gets up, grabs his friend, throws him in the mud, then walks away. In the background you see Belle's house. How could he fall down the stairs and what looks like over 20 feet away into the pig pen? (00:18:00)
Continuity mistake: In the dance scene, Chip is sent off to bed but comes back to watch. You can clearly see Chip's reflection in the floor, but he has no mouth. (01:02:25)
Continuity mistake: When Maurice looks into the castle after arriving, there are no yellow edges around the red carpet. Yet moments later, they appear.
Continuity mistake: When the Beast claws the painting of himself in human form, he tears his right eye. When Belle comes into the West Wing later, neither of his eyes are torn.
Continuity mistake: Just before the new "Human Again" song, Cogsworth is telling the other servants what to do. He is standing on the dog/footstool and tells everyone to clean up the castle. All the servants run off including the dog/footstool and Cogsworth falls down the stairs. A load of cogs fall out of him and Lumiere picks one up. It is quite small but as he dusts it off it grows and when he hands it to Cogsworth and it has grown in size again to over double the size it was when it first fell out of him.
Continuity mistake: When Gaston jumps from the rooftop in front of Belle and says "Hello Belle", he has four arrows in his quiver. However in all the previous and following shots, he only ever has three. (00:07:35)
Continuity mistake: In the Gaston song sequence, the villagers pick up the chair Gaston is in, and carry it in front of the bear rug. Then suddenly, Lefou is standing on the bear rug, and the chair crashes down on him.
Continuity mistake: When Belle approaches the West Wing, she sees a door and there is one statue holding candles next to it. However, when Belle runs out, there are statues on both sides.
Continuity mistake: When Mrs. Potts, Chip and the milk and sugar pot enter Belle's bedroom, they make some tea in Chip, who then hops over to Belle. After they pour and make the tea in Chip, as he hops off, in a closeup you can see the milk pot is standing beside Mrs. Potts and the sugar pot is next to the milk pot. Then when it cuts to a wider view, the milk and sugar pot have swapped places with each other. (00:30:30)
Continuity mistake: When Mrs. Potts puts Chip into the cupboard, saying "Into the cupboard with your brothers and sisters", between a wideshot and closeup, more teacups suddenly appear around Chip. (00:35:20)
Continuity mistake: When Le Fou and two other men chase the dog-like footrest, Le Fou has a pitchfork and the men have an axe and a sword. However when they bust through the kitchen doors, the sword has turned to a club. (01:13:10)
Continuity mistake: When Le Fou is chasing the dog-like footrest he is missing one shoe, which the dog has, but when Le Fou and the gang burst through the kitchen door, he has the shoe back. Then when the knives scare them away, it's missing again. (01:13:10)
Continuity mistake: When Lumiere and Cogsworth are persuading Belle not to go into the West Wing, in an overhead shot they're at the center of the red carpet, but in the next they're at the far edge of the carpet.
Continuity mistake: The large front doors to the castle open inwards, as seen when Maurice and Belle enter the castle for their first time. However when Belle makes her deal with the Beast, to stay with him forever, the Beast takes Maurice outside, however he opens the doors outwards. (00:13:05 - 00:23:05)
Continuity mistake: When Gaston is singing "Kill the Beast", he is scaring Lefou. Lefou has a bit of his white shirt visible underneath his coat. When he is climbing backwards up the stairs, this spot has changed to green. When he marches away, the spot is white again.
Answer: In the prologue, it states that the Beast must find true love by his 21st birthday, and that he had been cursed for ten years, so since his relationship with Belle takes place as the deadline approaches, he must have been 10 when he was cursed.