Imagine this: You are playing Starcraft, you are playing Protoss, you've been pushed back by leapfrogging units of Terran siege tanks but have managed to stop them at the gates to your main base. You got everything upgraded to throw out tons of troops in seconds if need be to reinforce or break out. Only the unit cap keeping it form being a wave, the same as the other guy. You're hoping he throws enough tanks at your defences that you can break out on the weakend numbers which is sure to happen soon when suddenly the game announces:
Nuclear launch detected.....
Nuclear launch detected.....
Nuclear launch detected.....
Nuclear launch detected.....
Nuclear launch detected.....
Nuclear launch detected.....
What? Six nukes incoming? But the Ghosts can't get close enough to the defenses to blow them apart! Then you see the little near invisible flashing red dots all over the back portions of your base!
Turns out the open space you were depending on to keep most units out wasn't covered and the terrans landed a dropship full of ghosts back where your troops couldn't see them. Then tanks were just penning you in until the nukes hit. Then they storm forward as the plan all comes apart....
I caused that OMFG moment to a friend and I still think it had to be one of the great moments in gaming for me since he tended to beat me most times.
It was also a very long game, me taking resource site after resource site, him fighting the whole way. If he had thought to defend the edges of his base against air attack where the open spaces stopped my tanks we might have been going at it for hours.