Polyploidy
Sometimes chromosomes can fail to separate during meiosis leading to duplication of entire genome (polyploidization)
Surprisingly, polyploidization is usually no problem for the cell. In fact polyploid plants are generally more vigorous than their parents.
Cells of genome-duped organisms are twice as large as cells of unduplicated ones.
Polyploids can’t mate with monoploids. This is in fact the basis of seedless watermelon.
Polyploidization in mammals doesn’t work so well because all but one X chromosome gets shut down