http://helpmewithbiblestudy.org/5system_Moses/dh13.aspx

While anachronisms are quoted widely, narrative discrepancies are also seen as evidence of post Mosaic authorship of the Pentateuch. Narrative discrepancies represent perceived textual inconsistencies, differences, or disagreements within the narrative. And some discrepancies can be attributed to copyist errors as discussed in this article The Reality of Copyists' Errors.

One common example is Genesis 21:14. In this apparent discrepancy, Sarah has Hagar expelled from the family when Ishmael is a young man around the age of 14 - 17. Destructive critics of the late 1800s based their criticism on the translation of the Septuagint version of Genesis 21:14:

And Abraam rose up in the morning and took loaves and a skin of water, and gave them to Agar, and he put the child on her shoulder, and sent her away, and she having departed wandered in the wilderness near the well of the oath. (Septuagint version – Gen 21:14)

From this translation, destructive critics ask how it is possible for Hagar to carry this young man Ishmael on her shoulders?

It is a valid question; however, the Septuagint is well known to have many corruptions in its text. The collection of scriptures that make up the Septuagint has a complicated and varied translation history including a Hebrew book translated more than once, revisions to particular Greek translations, and instances of modifications and amplifications. Thus, while it is worthy for study and consultation, it is not a manuscript used for the basis of today’s Old Testament translations.

The more accurate ben Asher text (Leningrad Manuscript B19a), used as the basis of many contemporary translations, indicates the translation of Genesis 21:14as:

So Abraham rose early in the morning and took bread and a skin of water and gave them to Hagar, putting them on her shoulder, and gave her the boy, and sent her away. And she departed and wandered about in the wilderness of Beersheba.(NASB – Gen 21:14)