The opening scene shows Cindy missing her two front teeth. The second scene, representing the next morning, shows her teeth full grown. Later in the movie, they are at different stages of partial-growth.
It is stated that the first ransom note was illegible, yet in the later shot of the crumpled-up paper, the writing is neat and easily-readable.
The mercs handle the ransom-note extensively with their bare hands prior to delivering it, so there would have been lots of fingerprints on it; the law-enforcement people would not have said that they could not find any prints on the paper.
Mitch states that the room which Linda shows him is a central room in the house with no windows to let interior light be visible from the outside at night, yet there is clearly a huge translucent-curtained french window right next to them, with its light liberally illuminating the room.
When the white dog "Tiffany" comes to the abandoned house for the first time, as she is climbing into the house, you can see a crew member down below spotting her as she climbs onto a high rail.
When Benji first enters the children's home on his own at the beginning of the film, a very obvious man in a white collared shirt opens the door.
A dog trainer is visible on the extreme right, snapping the fingers and twirling his arm, as Benji proceed to cross the street at the intersection.
Mitch is the worldly-and-law-savvy "brains" behind the operation, so he would surely have told his henchmen to wear latex gloves when writing the ransom-note and otherwise handling the stationery, to avoid leaving identifiable fingerprints on the paper.
Henry states that the ransom note needs to be written with the left hand to make it harder to trace through handwriting-analysis, yet he himself uses his own right hand to scribble the second note after he discards the one that Riley had tried to write.