Heterotopic ossification (HO) most commonly occurs at the amputation site if the amputation was performed thru the zone of injury, especially if the injury was a blast mechanism. Potter et al reviewed 213 military amputations and found heterotopic ossification was present in 63% of the residual limbs. A final amputation level within the zone of injury and a blast mechanism were risk factors for the development of heterotopic ossification. Symptomatic HO was removed successfully with a low recurrence rate (<10%). Other answers listed describe amputations at a level above the zone of injury.
Potter BK, Burns TC, Lacap AP, Granville RR, Gajewski DA. Heterotopic ossification following traumatic and combat-related amputations. Prevalence, risk factors, and preliminary results of excision. J Bone Joint Surg Am. 2007 Mar;89(3):476-86.
PMID:17332095 (Link to Abstract)