Haypiles and compost piles may self-ignite because of heat produced by bacterial fermentation. Linseed oil in a partially confined space can oxidize leading to a build-up of heat and thus ignition. Coal can ignite spontaneously when exposed to oxygen which causes it to react and heat up when there is insufficient ventilation for cooling. Pyrite oxidation is often the cause of coal spontaneous ignition in old Mine tailings. Pistachio nuts are highly flammable when stored in large quantities, and are prone to self-heating and spontaneous combustion. Large manure piles can spontaneously combust during conditions of extreme heat. Cotton and linen. When these materials come into contact with polyunsaturated vegetable oils (linseed, massage oils), bacteria slowly decompose the materials, producing heat. If these materials are stored in a way so the heat cannot escape, the heat build-up increases the rate of decomposition and thus the rate of heat build-up increases. Once ignition temperature is reached, combustion occurs.

Fire in coal mines
Forest Fire
Phosphorus ignites spontaneously on hot summers as its ignition temp. is only 35'C 
Oily rags often burst into flames spontaneously 
Burning of a matchstick spontaneously due to friction