The beginnings of stories allow for a little more leeway than the rest of the story for the simple fact that the beginning establishes the ‘conditions’ that the rest of the story operates under.

This is also why a lot of things that are usually not great to have in the middle of a story are okay in the beginning.

Examples of such may be freak coincidences which are generally not good, but are fine at the start of the story as they provide setup for the entire rest of the story.

Another example would be ‘unrealism’ or establishing the entire premise. Volcano, a rival film to Dante’s Peak, was built on the premise of a volcano suddenly appearing in the middle of Los Angeles.
Obviously, this is impossible on multiple levels: but the rest of the film is built on the premise of what if it did happen? Thus it’s at least half-way acceptable (though that doesn’t mean you can’t point out that it’s unrealistic).