The majority of sporadic carcinomas suffer from a kind of genetic instability in which chromosome number changes occur together with segmental defects. This means that changes involving intact chromosomes accompany breakage-induced alterations. Whereas the causes of aneuploidy are described in detail, the origins of chromosome breakage in sporadic carcinomas remain disputed. The three main pathways of chromosomal instability (CIN) proposed until now (random breakage, telomere fusion and centromere fission) are largely based on animal models and in vitro experiments, and recent studies revealed several discrepancies between animal models and human cancer. Here, we discuss how the experimental systems translate to human carcinomas and compare the theoretical breakage products to data from patient material and cancer cell lines. The majority of chromosomal defects in human carcinomas comprises pericentromeric breaks that are captured by healthy telomeres, and only a minor proportion of chromosome fusions can be attributed to telomere erosion or random breakage. Centromere fission, not telomere erosion, is therefore the most probably trigger of CIN and early carcinogenesis. Similar centromere-telomere fusions might drive a subset of congenital defects and evolutionary chromosome changes.