Mistakes, fails, and blunders that we make in the game.

in one of the games, I think against Hermit, I misclicked instead of upgrading the archers and deleted them.
Also, once I deleted a hero with all his troops and once with all the artifacts (somewhere +50 to stats in total).

Many probably had this happen: you're running through terrain with a movement penalty on wings, and for the night you switch to a cloak +2 to speed, and on the next turn you bulldoze through the rough terrain in the cloak.