Can be caused by radiation, certain chemicals, or transcription errors. Can be minor, such as a single wrong nucleotide, or major, such as entire segments of the chromosome being moved around. It's the caues of mutation and, indirectly, evolution. It manifests itself as Down syndrome, cancer, and all manner of bad things. It's probably worst to have it occur in stem cells and germ cells, because it can be propagated even if it's not cancerous. In a somatic cell, it dosen't propagate far, because somatic cells don't (generally) divide much.