I kinda have been defeated by CS before...Not really a stupid mistake though
It was in G&K, I had been trying to win a Deity game for a long time but most of the time I was defeated by like turn 100.
Now this was the first game where things actually were looking pretty good.
I started out as the Maya, had taken over China, some parts of Rome and we were in the Industrial era.
But then Russia(in the west) declared war on me.
All was going well, I even took a city of her.
Then Catherine decided to ally 2 city states that were east of my empire.
Needless to say almost all of my army was in the west trying to fight the technological superior Russians.
Because Russia had better units I couldn't pull my units back to the east.
So I tried to defend those cities with some units I had there and by building some.
But to no avail, those 2 CS took over and razed 3 of my cities and because I tried to reinforce there, I lost grip in the west..
And so I had lost the game....