Night of the Living Dead

Siblings Barbara and Johnnie visit their mother's grave in a remote Pennsylvania cemetery. During their visit, Barbara is attacked by a zombie. Her brother comes to her defense but is killed. Barbara flees the cemetery and discovers what at first seems to be an abandoned farmhouse. She seeks shelter there, only to find another pack of zombies. Shortly after, a man named Ben arrives, and the two clear the house of the dead and begin the process of barricading the doors and windows.
They discover other survivors who have been hiding in the cellar of the house: Harry Cooper, a loud and argumentative husband; his wife Helen; their daughter Sarah, who was bitten by a zombie and has fallen seriously ill; teenager Tom Bitner and his girlfriend Judy Rose Larson. The group is divided over what their next course of action should be. Harry believes everyone should retreat to the cellar and barricade the door to wait for the authorities. Ben thinks the cellar is a "death trap" and that they would be better served fortifying the house, which at least has alternate escape routes, and Barbara suggests that the group should simply leave the house on foot after she notices the zombies' limited mobility. An argument between Ben and Harry leaves the Coopers in the basement to tend to their ailing daughter, and the remaining members remain upstairs to continue their work reinforcing the doors and windows. The loud construction attracts even more zombies to the farmhouse, soon collecting a large mob.
The group devises a plan to escape using Ben's truck, which is out of fuel, by refueling at a locked gas pump a couple hundred yards away. A search of the corpse who lived in the farmhouse produces an unknown set of keys. Judy Rose, Tom, and Ben proceed up the hill toward the gas pump, but their plan begins to unravel when Ben falls from the bed of the truck and is left to defend himself. To their horror, the key to the gas pump is not among the bunch they brought with them. When Tom shoots the lock off, the leaking gasoline is ignited by a burning piece of wood in the truck. The resulting explosion kills both Tom and Judy.
Ben returns to the house to find things beginning to dissolve into chaos. Harry has wrestled Barbara's gun away from her and is now armed. Unknown to the survivors upstairs, the Coopers' daughter Sarah has died from the bite on her arm and has transformed into a zombie; she attacks and bites her distraught mother, who does not defend herself. When Sarah makes her way upstairs she triggers a shootout between her father, who is trying to protect her, and Ben and Barbara, who are trying to protect themselves. Both Ben and Harry are badly wounded; Barbara shoots Sarah. Harry retreats upstairs to the attic, while Ben makes his way to the cellar, where he shoots a reanimated Helen. Ben gradually goes into shock, and after realizing the gas key has been in the cellar the entire time, he laughs mindlessly at the irony.
Meanwhile, Barbara leaves the house alone and attempts to find help. She eventually joins a group of Countryside locals who are clearing the area of the undead. She's awoken the next day and is surrounded by safety of the media and townspeople. Noticing hillbillies playing around with a couple of zombies, she comments on the similarities between the living and the undead. She returns to the farmhouse to find Ben, who died of his wounds and has been reanimated. He gazes at Barbara before being shot in the head by one of the posse members. When Harry emerges from the attic alive, Barbara kills him in a fit of rage for causing Ben to die, and turns to leave the house, telling the vigilantes they have "another one for the fire". The film ends as Barbara hypnotically watches the bodies being burned in the fire.

Question: Why is the group's plan ruined?
Answer: Ben falls out of the truck bed