Aside from opening Piety. 
This thread is about silly things you've done in game that you didn't mean to do, and kicked yourself for doing afterwards.
Polish Barbarians - Had a worker mining salt for me a couple of tiles from the city. A Polish Warrior wanders up. I check my diplo screen, note he's friendly at the moment, and that he hasn't sent a whole horde over. Odds are this is just a wandering unit, not a war force. So I leave the Worker there and don't bother moving over a bodyguard unit. Next turn, the barbarian warrior captures my worker. Lesson learnt - next time look more carefully at what colour the unit is!
Forgotten Elvis - Intending a tourism victory, and I have a trio of Great Musicians hovering near the French border, with the intention of "invading" with them and carrying out some (presumably militarised) concert tours. Next turn France declares war on me, and I hustle my forces to defend my capital, leaving the Great Musicians on Alert where they are. Fifteen turns later, a shame-faced Napoleon offers me massive reparations and a peace deal, which I gleefully accept. Guess which units I forgot to move?

This thread is about silly things you've done in game that you didn't mean to do, and kicked yourself for doing afterwards.
Polish Barbarians - Had a worker mining salt for me a couple of tiles from the city. A Polish Warrior wanders up. I check my diplo screen, note he's friendly at the moment, and that he hasn't sent a whole horde over. Odds are this is just a wandering unit, not a war force. So I leave the Worker there and don't bother moving over a bodyguard unit. Next turn, the barbarian warrior captures my worker. Lesson learnt - next time look more carefully at what colour the unit is!
Forgotten Elvis - Intending a tourism victory, and I have a trio of Great Musicians hovering near the French border, with the intention of "invading" with them and carrying out some (presumably militarised) concert tours. Next turn France declares war on me, and I hustle my forces to defend my capital, leaving the Great Musicians on Alert where they are. Fifteen turns later, a shame-faced Napoleon offers me massive reparations and a peace deal, which I gleefully accept. Guess which units I forgot to move?