mutations in the mitochondria can slip through the quality-control checks unnoticed and therefore build up to high levels, if these mutations are harmful in their effects on males but not on females. This is because all of the screening of mitochondrial mutations is done in females as a result of their maternal inheritance.
One area where this seems to happen is in mtDNA's impact on male fertility. According to the journal article, P. Innocenti, E. H. Morrow, D. K. Dowling. "Experimental Evidence Supports a Sex-Specific Selective Sieve in Mitochondrial Genome Evolution." Science, 2011; 332 (6031): 845 DOI: 10.1126/science.1201157, mtDNA mutations may be a major source of the inferility found in about five percent of men.