What kinds of items should not be put into a sink waste dispenser?
From the operation manual of the most expensive Insinkerator:


  This product is designed to dispose of normal household food waste; inserting
  materials other than food waste into disposer could cause personal
  injury and/or property damage.


Not a surprising disclaimer.


  Do not put the following into a disposer: clam or oyster shells, caustic
  drain cleaners or similar products, glass, china, or plastic, metal (such
  as bottle caps, steel shot, tin cans, or utensils), hot grease or other
  hot liquids.


Why no hot grease though?


  Do not dispose of the following in the disposer: paints, solvents, household
  cleaners and chemicals, automotive fluids, plastic wrap.