31 years ago, on September 10th 1976, an Inex Adria DC-9 and a BA Trident 3B collided over ZAG VOR (Vrbovec, Croatia). All 176 people on board both aircraft lost their lives in the Europe's worst mid-air collision to date. The combination of non-standard ATC phraseology, controller work stress that contributed to a lack of coordination, and a lack of controllers themselves and modern equipment, are seen to have been the major contributing factors.

http://en.wikipedia.org/wiki/Zagreb_mid-air_collision