Corrected entry: In the scene when the party comes up from the underground base you can see the lights from a city that still lives, in the background. (00:34:30)
Revealing mistake: As the gang drives up to the space-ship diner, cars are whizzing by in the background. When they are driving to the base there's also a clip of a fire truck going down the highway in the distance.
Question: In this episode, several teams of human "harvesters" wander through a greenhouse filled with vegetable-hybrid zombies, searching for specific seed pods that might cure the zombie virus. None of the harvesters survive very long inside the greenhouse, even though the translucent plastic/fiberglass greenhouse walls are never more than a few steps away. In fact, the prized "Batch 47" is discovered just feet away from the translucent wall. Why couldn't the harvesters escape from the zombies by simply charging straight through the flimsy greenhouse walls? Or, even better, why didn't they just tear out the flimsy walls from the outside and search for Batch 47 from around the perimeter? I mean, it's just a greenhouse, not Fort Knox.
Answer: When the plant zombies would attack they would wrap people with the vines and hold them, so they couldn't escape.
Correction: The characters mention that the city looks like it's alive and lit up. Part of the story, not a mistake.