First place I (we) was betrayed at. A tribe member whose name I shall not mention was invited to the village. We went on to fight a bear for the first time - that's when Chief popped out and killed us all. (Well, not me, being the crafty, slimy ***** I am, but many others including Sunari.)
Spatsville! Second time we were betrayed. This time it came as a much bigger shock, since the betrayer had lived with us from day 3 or so. He got bad conscience and decided to give back a little of what he'd stolen (pretty much everything, he had made a cart and put all of our chests in it), but even then, it wasn't more than a few iron bars and some food he ended up giving us.
Finally, World's End. This is where I finally lost it. [Redacted], a very close friend of someone I knew well, stole more or less everything we had and used it for himself. They say third time's a charm, and apparently, so it was for me as well. We caught him bare-handed in the middle of his act, trying to transfer all of OUR creations to his bank or something. I finally snapped and decided that if I betray people first, they will not have any chance of betraying me.
Finally, the biggest and perhaps only reason I regret my actions, Sunari in a nutshell:
Surely none could outmatch Sunari's productiveness.
Either way, your turn! Post screenshots that are months or even years old.