If the initial reports prove correct, the fatal crash of Pakistan International Airlines flight 8303 on Friday may have resulted from the pilots missing a key line-item in their landing checklist:
On May 24th 2020 Pakistan's media quote a CAA official speaking on condition of anonymity that the aircraft made two attempts to land. During the first approach it appears the landing gear was still retracted when the aircraft neared the runway, the pilot had not indicated any anomaly or emergency, emergency services thus did not respond and did not foam the runway as would be done in case of a gear malfunction. The marks on the runway between 4500 feet and 7000 feet down the runway suggest the engines made contact with the runway surface, it is possible that the engines were damaged during that contact with the runway surface leading even to possibly fire.
On May 24th 2020 a spokesman of the airline said, the landing gear had not been (partially or fully) lowered prior to the first touch down. The crew did not call out the standard operating procedures for an anomaly and no emergency was declared. Most likely the crew was not mentally prepared for a belly landing and went around when they realized the engines were scraping the runway.
Gear-up landings do happen, though very rarely. Airports have procedures for mitigating damage and loss of life in these situations, starting with foaming the runway and having emergency equipment standing by. That neither pilot of PK-8303 called "mayday" until they had already destroyed their engines, and that the plane had an extreme nose-up angle of attack in its final moments*, suggests a serious training issue.
* The high nose-up angle would have increased drag and shortened the plane's glide to the airport. The A320 probably has a best-glide AoA around 4° nose up; anything higher, and the plane will slow down and lose altitude more quickly.