Results depend on what the substrate is, snake's size and how much is swallowed. A few grains of sand or a bit of leaf is harmless; it passes through and is expelled with the feces.
A web search with "sand impaction leopard gecko" (minus the quotes) as keywords will get you a lot of hits.
Corn cob bedding has rather large pieces that swell when moisture is added. That can cause fatal impaction if swallowed.
http://www.nbcnews.com/id/13939272/n...lanket-python/ has a piece about a Burmese python that swallowed an electric blanket.
Once, years ago, I opened up a friend's dead California king snake. Substrate was kitty litter. The only thing I could find wrong was a 2 mm piece of kitty litter in the lung. IMO, inhaling that was the cause of death, and I crossed kitty litter off my list of acceptable substrates.
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