This was first edition D&D and the characters were arranged in a circle with fighters to the outside and spellcasters on the inside. They were fighting a mob of demons of various types and levels. The thing is there were enough characters to form a tight circcle, but they just weren't doing it. There was just enough space between each of the fighters in the outer ring that the demons were trickling through. I remember my frustration when the players set up the ring, just knowing they were going to get their butts kicked because they tried to control just a little too much ground. What could have been an easy fight was now potentially a TPK. I also remember a round or two into it, thinking the outer ring wouldn't last one more round; death was coming.
...and then the party leader yelled out for the fighters to bring it in a little. He then adjusted all his fighters backwards about 1" each, making for a decent circle big enough to protect the casters and small enough to seal off the gaps. The next player did the same as did the next (we were running 2 segments at a time, staggered between the characters and the enemy. Saved, I thought, this would take care of it; it would be tricky but they might actually pull it off.
Until the last player stepped up. He proceeded to take all of his fighters and turn them around. They ran at full speed to the center of the circle and then turned around again. So, we now had a tight ring of fighters in the middle - standing behind the spell-casters - and another ring of fighters on the outside, the latter ring now looking like swiss cheese. And with the last player done, I now ran the enemy for 2 segments.
...TPK.