Never had this issue. I bred leopard geckos almost a decade ago, and did keep the adults on sand--never had an issue with them, either.

Impaction is where the foreign particulate substance is built up to the point where it is blocking the intestinal tract, and will not move. It is a form of intestinal blockage.

Sand impaction isn't specific just to reptiles, but other animals may, for similarly inexplicable reasons, decide to eat sand as well.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1482454/

I wouldn't keep leopard geckos on sand today, but some species still require particulate substrates (such as Nephrurus). Some reptiles DO live on sand in the wild, and one would think are unlikely to engage in this self-destructive behavior.