Oh yep i also did that. Probably my most stupid mistake. So i was playing GnK culture with Egypt and my southern neighbor was Rome. Of course i had a bunch of wonders going tradition with Egypt , so after some BS backstabbing from Ceasar i thought ''Ha , now go home you whiner i'll take that city off your hands , sure" Clicked accept , and gloomy dramatic music starts playing.... ''Awwwww wtf''.My dumbest decision was accidentally accepting a peace offer which involved giving away a few of my cities to the AI. I was winning the war, mind you, and when the screen popped up I had a brain fart and thought that the AI was offering me his cities in exchange for peace. Needless to say, I had to reload (this is why I make the game do an autosave every 4 turns instead of the default 10)
The mistake I make at least once a game: partway through building a crucial national wonder, I cash buy a settler and plant a new city.
I recently walked an undefended Settler next to a Barbarian camp at turn 3, instantly losing the game.
Toss-up between 2, both dealing with the same mistake: in G&K (don't know about BNW), those little notices bubbles that pop up on the right side of the screen between turns. If you click on them, they center the screen on the event on question (such as a new great person) but don't actually select the unit. As such:
1.)I built my first caravel, and right-clicked on the dark section of the mini-map, about 10 turns later realized I was exploring the uncharted waters with an embarked great scientist.
2.)Great Merchant is born, I click on the bubble on the right side of the screen, right clicked on a CS's territory, yeah, trade mission. Except I didn't send the Great Merchant... sent the frickin' SSengine.
I recently walked an undefended Settler next to a Barbarian camp at turn 3, instantly losing the game.