Continuity mistake: After Leon rescues Mathilda from the SWAT team and dodges the snipers, he shoots out a ventilation fan to make an escape route. He gets off two shots before we see the vent, but when it does we see about 7 bullet holes around the vent. Then the camera goes back to Leon as he continues firing. When the camera switches back, the holes around the vent are different. (01:27:05)
Continuity mistake: In the very first scene where Old Tony is debriefing Leon, we see Leon pick up his glass of milk and drink the entire glass. However, if you look at the reflection of Leon's glasses as he takes the glass away from his mouth, you can see that the milk is not finished. (00:02:30)
Continuity mistake: Before the dreadlocked man starts looking at the vinyl records in the apartment, he places his uzi down with the butt facing the camera. In the next shot the barrel is facing the camera, and switches back again in the next shot.
Continuity mistake: In the scene where Mathilda returns to her apartment and takes the hidden stash of money out from the loose floor board, she leaves the board open and also takes the money out of the plastic bag and leaves the bag on the floor too. In the next shot, the camera angle switches to her looking down the hall because she hears people coming. When the camera switches back to the original angle of her scurrying away, the floor board is back in place and she's wrapped the money back up in the plastic - nowhere near enough time to do that. (01:02:25)
Continuity mistake: When Mathilda hides the cigarette from Leon it's practically new, but in the next shot (when Leon asks her why she hid the cigarette) it's almost finished. However, in the next shot it's practically new again.
Continuity mistake: When Mathilda spends her first night at Leon's apartment, Leon places a messily folded blanket on her in one shot. The camera then goes to a different shot of Leon, and then back to the original. However, we can now see that the blanket is mostly unfolded and placed neatly on Mathilda. ..... (and little pink doll she is holding changes position). (00:37:15)
Continuity mistake: At the beginning of the film, the intended target calls 911, with the phone's aerial fully extended. The shot later it's half the length. (00:07:00)
Continuity mistake: Leon is playing with Mathilda and she can't guess the actor. she tilts her head and then angle changes to the back of her head. But now not only her head is not tilted but when she says "I swear, I was going to say that actor" she is not mouthing the words.
Continuity mistake: When Leon shoots out a ventilation fan to make an escape route he stands right in front of a window. In the previous scene snipers were shooting at him, why are there no snipers shooting now? (01:26:20 - 01:27:10)
Continuity mistake: In the scene where Matilda is expressing her feelings to Leon, she lies on the bed and her hair falls in two separate strands, by the next shot (although she has been motionless on the bed talking), her hair is neatly spread.
Continuity mistake: Mathilda's school switches from being in Wildwood, NJ (at the start) to being on Roosevelt Island, NY. (00:18:15 - 01:38:15)
Continuity mistake: In the scene where Leon shoots out the ventilation shaft, you can see bullet holes through the wall, but when he starts to use the axe on the wall, in the next shot from inside the ventilation shaft the bullet holes have disappeared.
Answer: No specific reason is given; it's most likely intended to highlight that her family are bad people (they are drug dealers after all) and to show in contrast that Mathilda and her little brother are not, so we feel badly for them.
Purple_Girl