A coin can be damaged in a million different ways while in circulation, without being present when the damage is taking place, you don't really know the true cause or the intent. There are a number of tell-tale signs of specific damage though, which once you see it you can pattern match in your brain similarities. Some important points to remember:

Copper-Plated Zinc Cents

Bronze Cents

Zinc-Plated Steel

Copper-Nickel-Plated Copper

Copper-Nickel Alloy

Silver-Copper Clad

Silver-Copper Alloy

Sanding

Polishing

Filing

Snips or Shears

Vice

Mounting - Bezels

Jewelry Holes

Antique Coin Savers

Spooning

Washers

Shot

Plating

Coin Art

Cleaning

Texas Cent

Counterfeits

Fire and Heat

Soils

Water

Dryer

Water Fountain

Fuse Box

Cup holders

Parking Lot

Glue

Shelac