Although the existence of these terrible creatures is arguable, many writers would agree that they are a terrible plague to the craft. Small, devious little critters, Typo Demons grow from crumbs etc. dropped between the keys of a keyboard or typewriter.
While typing, many will attack at once, causing horrific misspellings and grammatical errors. They do this by jumping onto keys one is about to use and deflecting your finger onto an adjacent letter; the strongest can cause a mistake that involves letters on entire different sides of the keyboard.
They are an extreme nuisance to literary expression, and can result in sentences like "I am feeling sick" turning into "I am feeling d..." (true story) and so on.
These creatures are tiny, invisible beings that seem to live in every keyboard around the world. Devices and training have been implemented to stop them, but many people, especially youth, are still susceptible to their effects.
The only way to avoid them is patience and care while typing; typing under the influence of any mind-altering substance is not recommended, as this is a vunerable time for them to strike.