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

Coin Rolling Machine

Sanding

Polishing

Filing

Snips or Shears

Vice

Mounting - Bezels

Jewelry Holes

Antique Coin Savers

Spooning

Washers

Shot

Plating

Coin Art

Cleaning

Texas Cent

Counterfeits

Soils

Water

Dryer

Water Fountain

Fuse Box

Cup holders

Parking Lot

Glue

Shelac