Losing with an army several times the size of the enemy.
It was an early mission. I sent a large army to attack a relatively small army. It was 25 vs 8 or so. About 4 or 5 of my units actually fight and the rest actually just stay still beside the enemy. Even when they were on the square beside them! What I shouldn't have done was sent my hero in the battle as I lost him and had to restart. One time I wasted all the resources so I had to restart too. Another time I sent an army of air units thinking that since air units are not obstructed by terrain or units (they can fly past each other) I thought I would win. Same problem when the units actually fought. Like 80 percent of my air force was just standing still during the battle and all of the enemy units were all attacking. I sent my hero in that round too and died. Maybe it was because I used group select to attack the base in a large mob, but that is no excuse if it is.