What would be an example of a decomposition reaction

  An example of spontaneous decomposition is that of hydrogen peroxide, which will slowly decompose into water and oxygen: 

2 H2O2 → 2 H2O + O2 
Carbonates will decompose when heated, a notable exception being that of carbonic acid, H2CO3. Carbonic acid, the "fizz" in sodas, pop cans and other carbonated beverages, will decompose over time (spontaneously) into carbon dioxide and water 

H2CO3 → H2O + CO2 
Other carbonates will decompose when heated producing the corresponding metal oxide and carbon dioxide. In the following equation M represents a metal: 

MCO3 → MO + CO2 
A specific example of this involving calcium carbonate: 

CaCO3 → CaO + CO2 
Metal chlorates also decompose when heated. A metal chloride and oxygen gas are the products. 

2 MClO3 → 2 MCl + 3 O2 
A common decomposition of a chlorate to evolve oxygen utilizes potassium chlorate as follows: 

2 KClO3 → 2 KCl + 3 O2

If you eat to much it could kill you.