Tin pest is a chemical reaction affecting tin at low temperatures which turns pure tin from its usual smooth, silvery appearance to a dull grey powder. This change in appearance and chemical properties is caused by the transition of tin from one allotrope to another. Allotropy is the property of many elements to have more than one chemical form depending on how they're bonded. Carbon is probably the most well-known allotropic element—depending on how its atoms are arranged, pure carbon can take the forms of graphite, diamond, buckyballs, or carbon nanotubes. Like many compounds, grey tin is self-catalyzing, that is, the presence of a seed crystal of grey tin will accelerate the process of tin pest. Since grey tin is a powder, this often means that tin pest rapidly spreads as particles of grey tin are spread.
Because tin pest essentially disintegrates tin, it has posed problems throughout the history of human civilization. Though tin use has declined in the modern age, it used to be highly valued for its easy workability as well as its relative abundance. In the middle ages, tin was often used for the pipes of church organs which were observed to be prone to rapid deterioration in colder climates. There are also many stories which feature tin pest throughout history that may or may not be true. In one a Russian tzar kept a cellar full of tin for his army, only to discover that it had turned to dust during the winter. Other stories attribute the failure of Napoleon's invasion of Russia to the tin buttons on his soldiers uniforms, the buttons literally decomposing and exposing the French army to the elements of the Russian winter.
In the modern era, tin pest has become increasingly important with the rise of consumer electronics and the recognition of lead as an environmental hazard. While most solder used to be composed in a large part from lead, regulations in the last half century have replaced it with tin. Tin pest in a single connection can disable an entire electronic board by first turning to powder and then re-melting when heated, creating short circuits.
You can watch of video of tin pest here.