Final answer:
A diploid organism with some polyploid cells is experiencing polyploidy, particularly autopolyploidy if the extra chromosomes are from its own species, often due to meiotic errors. This condition is relatively common in plants but rare and usually problematic in animals.
Step-by-step explanation:
A diploid organism that has some polyploid cells is exhibiting polyploidy. Polyploidy is a condition where a cell or an organism has extra sets of chromosomes. In the case of autopolyploidy, these extra chromosome sets are from the organism's own species. This situation arises from an error in meiosis, during which the chromosomes do not separate properly into separate cells. While polyploidy is rare in animals, often leading to sterility due to meiotic irregularities, it is much more common and beneficial in plants, resulting in larger and more robust individuals.