When chromosomes lie on the metaphase plate, they have the characteristic X shape. But these are actually two chromatids that are held together at the centromere. If separation fails and both chromatids are pulled in the same direction it leads to trisomy (three homologues chromosomes). So when exactly does a chromatid become a chromosome if it's not after successful separation?
If no centromere split occurs during anaphase and consequently chromatids are pulled to one daughter cell, the possible outcome is during telophase the chromatids uncoils into chromonema which in subsequent divisions become supernumerary chromosomes.