We tend to be a bit over zelous sticking with our 50 and 60 and 100 year 'offical secrets' rules. That said the slapton sands incident has been the subject of many TV documentaries even in the absence of official acknowledgement up to know. There were that many witnesses there is little doubt it happened.
Tragic - but that's war. My own grandfather told me he saw more men lost in training accidents than he did in combat.
And I once read that in World War One, 25% of planes were downed due to mid-air collisions during dogfights, as often with friendly planes was with the enemy.
Another tragic military incident related to D-Day that was hushed up, and to this day, has received little mention. Early 1944 a large airborne drop exercise off the panhandle of Florida went astray. The night time exercise, under full moon, was intended to drop paratroopers on either St. George Island or Dog Island (not sure which). The idea was to make the drop in the direction along the longest axis of the island (east-west) ... which gave the paratroopers the largest landing zone. The aircraft approached the island drop zone at a right angle, north to south, resulting in numerous (believed to be in the hundreds) of paratroopers landing in Apalachicola Bay and drowning. True story related to me by a WWII vet who commanded one of the rescue (recovery) craft dispatched from Tampa Bay to help in the recovery of bodies.