There are examples of foreshadowing peppered throughout Ray Bradbury's "The Veldt." Foreshadowing is basically a hint as to what will happen later in the story.
"The Veldt" opens with foreshadowing. In the opening dialogue, George and Lydia Hadley have a conversation about something being wrong with the nursery. She suggests calling a "psychologist," not some repair man, to look at the room. This suggests that the nursery has a mind of its own. The problem with the room is not one that can be fixed by a wrench or by replacing a bulb.
Another example of foreshadowing occurs when George and Lydia step in the room and look off in the distance and see lions eating "some animal" and then hearing a scream. This scream is mentioned several times and later turns out to be the scream from the parents before they are eaten after the nursery comes to life.
A third example of foreshadowing is the drastic way the kids will respond to George and Lydia's decision to shut down the house for a few days. George says that last time he tried to lock the nursery, "the tantrum he threw! And Wendy too. They live for the nursery." When the parents shut down the room, the children plot to have them locked in and eaten, which is what happens.
Finally, the names of the children themselves can be examples of foreshadowing. Wendy and Peter are clear allusions to Peter Pan. In Peter Pan, the children are unhappy at home and leave their parents in favor of a magical world where they can fly. The children in "The Veldt" follow the same path, albeit in a violent way, and abandon their parents in favor of a magical world.