These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Which doesn't mean Ebert's (and other critics') problem with the movie is entirely unjustified, either - reality often doesn't make a good story, and filmmakers need to know when to diverge from reality to make the story more engaging.
Sometimes the length of Bethany's stump changes throughout the movie. The stump was gotten by photographing AnnaSophia wearing a green sleeve on the relevant arm and then erasing that arm in post-production. The real bite was almost up to the shoulder.