And Abraham journeyed from there to the South, and dwelt between Kadesh and Shur, and stayed in Gerar. Now Abraham said of Sarah his wife, “She is my sister.” And Abimelech king of Gerar sent and took Sarah. (Gen 20,1-2)

We don't know what caused Abraham to move from the heights above Sodom to Gerar, but it was very likely the dry season. Indeed, it is mentioned later in chapter 26: "there was again a famine in the land, different from the days of Abraham" (26:1). Here he also goes on to describe the disputes that Isaac had over the wells that Abraham had dug there. The area of Gerar was a lowland, only 40-50 m above sea level, and apparently abounded with what the highlands did not have in the dry seasons: water.

If we want to be critical of anyone, we can always find a reason, but finding one now against Abraham is really extremely easy. Because of his half-truth, Sarah found herself back in another man's harem. Is it at all possible that a man of God acted in this way, but moreover one who had already experienced a similar thing, and it was a memento with so many implications for his later life that he must have remembered it very well?

But as Oswald Chambers once said: Let us not overconfide ourselves and say of anything too easily: I will never do such a thing again! Not even the best of us are safe from falling. For we cannot be sure just because of what (we think) we are. The foundation of our security is not in us. It is always and only the grace of God that allows us to remain on our high places. Our task is to remain on them and not to leave them of our own free will. Then we have victory.