Continuity mistake: When we first meet the three blonde triplets, the one in the green dress has the Ariel-like bangs, but in the beginning of the Gaston song she has swapped hairstyles with her sister in the orange dress. When Gaston starts singing about eating eggs they switch back, but during the last shot of the song they're both sporting the red sister's hair.
Continuity mistake: When Mrs. Potts is singing during the song "Be Our Guest", the spot doesn't actually appear on her until she notices it.
Continuity mistake: When the household items fight the mob, six beermugs fling tomatoes at a man. It cuts and there are suddenly just five beermugs. (01:12:15)
Continuity mistake: Between shots of Gaston throwing the bag of gold on the table and the man taking a coin out, the bag opens by itself.
Continuity mistake: When the song Be Our Guest ends, in one shot Belle's right arm is on the armrest, but in the next she is suddenly clapping her hands.
Continuity mistake: When Mrs. Potts and Chip go to Belle's bedroom, the small colorful decorative pillows that lie on Belle's bed disappear, then reappear, etc., and the three large pillows change their appearance and position in the following shots as well. (00:30:20 - 00:31:05)
Continuity mistake: When Cogsworth comes into the study to say that Belle isn't coming for dinner, he opens the door inward and doesn't shut it. In the next shot of the doors from the hallway, though, the doors are closed, and the Beast opens both doors outward. They aren't double-swing doors like the kitchen ones. (00:32:30)
Continuity mistake: When we first see Belle at the beginning of the movie, there's a bridge in front of her house, which she crosses over to get to town. But later, after Gaston plots with the insane asylum keeper, there's a wide shot of Belle's house with a horse and buggy waiting in front, and the bridge is gone.
Continuity mistake: While Lumiere is seating Maurice in front of the fire, Cogsworth tumbles down the stairs and his nuts and bolts go flying out on the stairs. The camera cuts to Maurice, then back to Cogsworth on the stairs, but all the nuts and bolts are gone. (00:14:40)
Continuity mistake: When Gaston enters the West Wing with his bow, the painting on the right side of the door is different than it is at all other points in the movie.
Continuity mistake: When Mrs. Potts tells the Beast, "Straighten up. Try to act like a gentleman." the table she is on has nothing on it. Then when she says, "But don't frighten the poor dear.", two glasses and a batch of grapes has appeared.
Continuity mistake: In the scene where everybody's singing about Gaston, the legs of the chair he's sitting on change between shots.
Continuity mistake: When Maurice looks up at the castle after he has just arrived, a lion's head is carved over the front door. But when Gaston leads the villagers to the castle later in the film, the lion's head has been replaced by a green shield.
Continuity mistake: When Lumiere suggests to see "The gardens or the library" his flames are at his sides. Then when Belle replies "You have a library" his candles are at his sides.
Continuity mistake: The cracks on the mirror seen while the Beast is taking a bath change between shots.
Continuity mistake: After Belle refuses to go to dinner with the beast, he bursts into the West Wing and we can see the hallway outside the doors. It's completely different than when Belle comes down it later.
Continuity mistake: When Belle sits down on a fountain to read the blue book she got from the librarian, some sheep pass by and one of them bites a piece of the page off. But in the next shot, when Belle sings "But she won't discover that it's him til chapter three.", the page is intact again.
Continuity mistake: After the wolves run away the Beast is shown standing in the snow before collapsing and there are five cuts on his arm. However, when Belle treats the wounds in the next scene, there are only three cuts on his arm.
Continuity mistake: In the 'Human Again' song, near the end of the song Cogsworth and Lumiere turn on the fountain and the large middle statue sprays out water, but the two statues either side of the middle one are turned off. Then the wardrobe jumps out onto the balcony singing and then it cuts to an ariel shot, and now the two side statues are suddenly spraying water too. (00:58:45)
Continuity mistake: When the first song ends, Belle has left the village behind and all the crowd watches her ten meters away. In the close-up and the following scenes with Gaston, she's now very close to the crowd plus a baker with a wheelbarrow has suddenly appeared.
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.