Yes, you read right. Greatest Failures, stupid deaths and foolish actions are all welcome here. I myself have several failures in my group which are posted on some of the other topics but here is one that is new.
My group was doing an Exterminatus mission where they had to find the enemy base and put a sensor there so the fleet could bomb the base. The leader of the group watches too many Vietnam war movies and plays too much Call of Duty so he expected a small wave of bomber, nothing Space Marines can't handle. He thought that the group could weather the exterminatus and leave after it. He was sorely surprised when lance strikes, barrage bombs and gas hit his position. Nobody survived after that.