Corrected entry: When Grandma is being carried through the woods (after walking the boat through the acid lake), the far shot shows her to have shoes on. The close-up, however, shows that she hardly has legs, much less feet.
Corrected entry: It's amazing how the helicopter flew to the town in the ash without any problems, then suddenly gets its engine shredded from the same ash.
Correction: The danger for the helicopter is that ash could block the air filter, overheating the engine and wrecking it; moreover, the heli was quite overloaded and its engine had to work harder with a cooling system that didn't work well or did not work at all.
Correction: Look at the shot of her legs frame by frame - if you stop at one of the frames right before the camera cuts back to Rachel, you can clearly see that Ruth still has legs - they're just bloody and raw from the acid. The way the fabric of her pants is gnarled, when seen at full speed the camera moves and cuts away so fast that it does give the impression of missing limbs.
Chris Moyer Grice