Starting with the original accident here, this has to win the "Most avoidable accident " award. Sitting in a 30 foot hover waiting for the snow cloud to envelop you, with the Number 2 creeping up behind you only has one outcome, and this was it. They either didn't know the correct technique, or forgot they were over snow. Attempted formation zero-zero landings is probably an advanced lesson...
So, the Norway stuff for the UK.
The Puma incident was a classic. Puma enters whiteout, loses references, crashes and rolls over. Army or RM Gazelle goes to assist, attempts to land, whiteout, rinse and repeat. A second Gazelle then tips up, attempts to land, whiteout, and now 3 helicopters crashed in the snow. I don't remember there being any serious injuries but talk about slow learners.
The 2 Flight AAC incident was at a civvy airfield in Norway (Floro?), transiting back from Winter Ops. 5 Gazelles (the entire Flight!) were hover taxiing in for a refuel, the area was a bit tight and the 2 front gazelles clipped each other causing bits of rotor blades and crap to fly everywhere, those 2 crashed immediately, the debris then hit the 3rd aircraft and the 4th aircraft over torqued trying to avoid the ****fest coming at him. The fifth aircraft was far enough back not to be part of the destruction. The 2 in the mid-air were Cat 5, third one was Cat 4 and the fourth was Cat 3. A Norwegian ground crew operative was seriously injured by the debris. Back to the "Most avoidable accident" awards...