Death of a Thousand Nibbles
The first incident is definitely my single greatest failure. As a bit of background, I had recently been exposed to stories like Tucker's kobolds, and really wanted to drive home to my players that even common, small creatures could be a threat to them. I introduced a crazy druid who was convinced that the player characters were responsible for heinous destruction of her forest. She sent an army of animals to attack and kill the party. The character that is relevant at this point is, well, let's call her a monk. It was the heady days at the end of AD&D 2e, and it was a custom class. She was capable of taking a large number of relatively weak unarmed attacks.
I was also in a mode of thinking that "fudging dice is bad" and "letting the dice fall where they may is the only way to provide true tension". However, this was fairly early in the campaign, and I hadn't really demonstrated that yet. To make matters even stickier, the player in question was my wife. I felt that I needed to prove that I was not biased in her favor.
So, as part of the larger encounter, a horde of bats attacked her character. At the time, I didn't have good rules for running a swarm. I made up some. Unfortunately, I got the math badly wrong. I'll skip the embarrassing details, but effectively she was taking d10 every round and it would take her about six rounds to kill them all. You are likely not shocked that she ended up dying. From bats.
It was a terrible encounter. They were terrible ad-hoc rules. It was probably the least heroic death you could imagine (other than my character that died from polymorph not once but twice). I'm pretty sure that everyone reading this can list a dozen basic GM mistakes here. But, I wanted to be a fierce and impartial DM. So, I made the death stand.
That definitely ranks as my greatest GM failure. But, oddly enough, not my most destructive.
Beware the Alpha Strike.
So, we were starting up another campaign a few years later. It was set in a decaying urban environment. This was the first adventure. Some dark creepers (humanoid creatures that lurk in the shadows and scavenge off the humans) had broken into the characters' house and stolen a strongbox. They chased them into the sewers. There were a couple good minor encounters, and then they came to the big set-piece encounter.
The tunnel opened out into a huge central well. Nearly a hundred of these creepers were clinging to the walls, armed with crossbows. They fire.
When I worked out this encounter, this was supposed to be an initial "oh shit" moment. What I knew was that they were terrible shots, with crappy equipment. Also, they were cowards. Every time one died, half the remaining creepers would flee. On paper, it would be a hard encounter, but easily winnable.
Unfortunately, two things worked against me. The party had split up, and only two characters walked into this ambush. Also, I failed to take into account just how devastating fifty incoming crossbow bolts, no matter how inexpertly fired, would be to a first-level character. Especially when they all targeted the same character.
I seem to recall that he was only hit by five or six bolts out of the massive hail of fire. But, that was easily enough to kill him.
Now, I had somewhat learned my lesson from my earlier failure. I was planning out ways to fix the situation. I didn't want to just ret-con it, but I had ideas on how to bring the character back. Because I did recognize that it was largely my fault as DM.
The player, however, was convinced that I had deliberately targeted and killed his character. He refused to talk to me about it. He also refused to make another character. The whole campaign sputtered and died.
Aside from some of the obvious lessons (like actually thinking through my encounters), one of the lessons I learned is that very few players actually like random death. They may be willing to go down in a heroic sacrifice, or when forewarned that they are walking into a suicide mission. But, they don't want to go down to the schmuck minions.
Also, players really hate it when the deck looks stacked against them, and then it turns out to actually be stacked against them. They want to turn the tables on the bad guys, preferably in a dramatic fashion. Slogging through an encounter where you just have to hope that the bad guy runs out of hit points first is unsatisfying.
Bringing this back to the opening paragraph, I clearly should have fudged the rolls in both of these situations. When I saw that the bats were killing the monk, I should have noticed that my ad hoc rules were no good and had the bats suddenly fail a morale check and fly away. When I realized that the incoming crossbow bolts were all going to be targeting one character instead of split between six as I had been expecting, I should have either had only some of them fire, or given him some chance to duck back into the tunnel. Letting the chips fall where they may cost my wife (and me) a really cool character, and cost me a potentially really cool campaign.