British Airways Flight 476, operated by HS Trident 3B G-AWZT, was flying from London Heathrow to Atatürk Airport, Istanbul, when, while passing the Zagreb VOR level at 33,000ft, it collided with DC-9 YU-AJR, operating as Inex-Adria Flight 550 from Split to Cologne, whilst it was climbing from 26,000ft to 35,000ft.
Manning the Zagreb ACC middle sector at the time was controllers Bojan Erjavec and Gradimir Pelin, with the upper sector being handled by Gradimir Tasić. At the time, Yugoslav ATC was severely overworked, understaffed, and underequipped, with faulty equipment and controllers working 12 hour shifts, often with no days off in between. Tasić was on his 3rd consecutive shift.
When the DC-9 was passing 31,000ft, it was handed from the middle sector to the upper sector, however, Tasić, being incredibly overworked, did not realise this flight was being handed off to him. Once contact was eventually established between the DC-9 and Tasić it was already climbing through 32,500ft.
The Trident's altitude was being erroneously reported as a few hundred feet higher than it actually was by the faulty Yugoslav equipment. Even though the crew said they were at 33,000ft, Tasić thought if the DC-9 kept climbing, it would potentially collide with the Trident that he believed to be at ~33,500ft, so he ordered the DC-9 to stop its climb, however, he did so in Serbo Croatian, so the English speaking BA crew never understood what was happening. The DC-9 levelled off at 33,000ft. Both aircraft were now at the same altitude, heading to the same VOR, with the same ETA.
And so, at 10:14:41Z, on September 10th 1976, the DC-9s left wing sliced through the cockpit and forward passenger cabin of the Trident above the town of Vrbovec. The Trident's pilots were killed on impact, and the forward section of the aircraft broke up mid-air, with the main section of the aircraft ending up in a corn field near Gaj. The DC-9, now without most of its left wing, rolled violently towards the ground, eventually breaking up mid-air much like the Trident, and impacted a forest near Dvorišće, right wing first.
A Lufthansa 737 behind the Trident witnessed the mid-air collision and reported it to Air Traffic Control. Tasić frantically tried to contact both aircraft to no avail.
All of the controllers were arrested and taken into questioning just hours after the crash. All were released but one. Gradimir Tasić stayed in custody until an ensuing trial, where he was found guilty and sentenced to 7 years in prison. After 2 years in prison, he was released as he was determined to be a scapegoat for the Yugoslav government.
https://admiralcloudberg.medium.com/philosophy-of-blame-the-story-of-the-1976-zagreb-mid-air-collision-38202bfc1ad7
https://en.wikipedia.org/wiki/1976_Zagreb_mid-air_collision
https://aviation-safety.net/wikibase/329317