In Dwarf Fortress, on my most successful fort Boldropes, once I breached HFS and the exact moment that happened: "A vile siege of darkness has arrived!"
Cue multiple facepalming at not having savescummed. I went out the dorfy way- sent my strong squad to the depths to contain the demons, and tried to erect a stalling path using hurriedly conscripted masons, then sent the other three squads to the surface to contain the siege. I left the fifth squad inside as general defense- some stationed nearish to the entrance as last line of defense, and some stationed near the stairway to HFS.
Goblin siege was broken, with gigantic casualties. The 'Diamond Gods' were down in the depths fighting a losing battle- all but the most powerful dwarves were swept aside in a few seconds. Iteb scored three kills before getting a melted head and falling unconscious- thankfully the demons left him alone while he was unconscious. Zefon scored no kills but multiple arm wounds. "Kurok" got no kills, no injuries beside a left arm that went necrotic from a sting.
I abandoned before Iteb and Zefon got killed- those two were my favourites and I wanted them to escape.
Minecraft: I was ambushed while outside by a creeper. It dropped me to one heart and ruined my burrow. I hid in the hastily-patched remains of my burrow until my health had regenerated enough for me to repair my burrow's entrance - by which time it was day. I crept out, repaired the burrow, got ambushed by another creeper and got dropped to half a heart. FUUUUUU-
Sprinted out to try and get back to the burrow's once again damaged entrance, regenerated my health very slowly because i'd gotten hungry, then when it was full again...
I descended to the depths of the chasm i'd colonised and promptly got shot to death by a skelly. RAGE.
Runescape: This is a historic one I remember. I was just about to do the final boss battle in the quest "Shadow of the Storm" and realised that I'd forgotten the crucial MacGuffin to defeat the boss with- I raged for a few seconds while Agrith-Naar (level 100) whittled my player (then level 76) down to 2/68hp and I hit my "EMERGENCY ESCAPE BUTTON" (cabbage-porting using my Explorer's Ring) and found myself in Falador on 1hp. So close. Healed, prepared again, walked the bloody ages to get back to the area I was in, and fought Agrith-Naar again. He demolished me in about four hits, bringing my character to 10hp / 68hp. I panicked and ran, then remembered that I had plenty of food. I ate, then played a war of attrition- Dragon Dagger P++ Special attack! 20-20! Ooh yeah. Run away and range with my crappy Maple Shortbow and crappy Iron Arrows. Yeah! 4, 4, 2, 0!
Then I ran out of food and figured "It's all or nothing now. Let's do this." Put my prayer on to protect myself from magic damage, charged in with Silverlight and got Agrith-Naar to very low hp. At which point for some reason I was hitting 0, 0 , 0, 0, 0. I kept hitting zeroes on Agrith until i got a lucky hit and defeated him.
I had seriously underestimated Agrith-Naar. I'd fought a level 172 Black Demon not long before that with very little trouble at all (of course, Black Demons only use melee so I could protect myself but Agrith used Magic and Melee.) RAGEEEEEEEEEEEE after I'd finished the quest at my stupidity.