Go on - own up. I bet everyone's done something really silly at some point while playing civ. And I want a laugh
FWIW, here's mine:
1. Switching to theology to get the XP in my units when I didn't actually have any religion in my main unit-building cities.
2. Razing a captured city coz I thought it was too close to one of my existing cities. Except I'd completely miscounted the squares separating them - there was actually no overlap at all in the two city radii. About 10 turns later I was somewhat sheepishly founding a new city on the exact same square as the razed one.
3. Founding a city in the early game specifically to take advantage of the perceived production bonus of 3 ivory in its radius. Except the ivory was on desert, the only other squares in the city radius were hills, plains, and sea. There was not a single square around capable of producing more than 2 food, and therefore no way to ever work the ivory squares without killing growth. (In my defence this was back in late November, I'd only had Civ4 for about 2 weeks. I don't think I'd make the same mistake now...)

FWIW, here's mine:
1. Switching to theology to get the XP in my units when I didn't actually have any religion in my main unit-building cities.
2. Razing a captured city coz I thought it was too close to one of my existing cities. Except I'd completely miscounted the squares separating them - there was actually no overlap at all in the two city radii. About 10 turns later I was somewhat sheepishly founding a new city on the exact same square as the razed one.
3. Founding a city in the early game specifically to take advantage of the perceived production bonus of 3 ivory in its radius. Except the ivory was on desert, the only other squares in the city radius were hills, plains, and sea. There was not a single square around capable of producing more than 2 food, and therefore no way to ever work the ivory squares without killing growth. (In my defence this was back in late November, I'd only had Civ4 for about 2 weeks. I don't think I'd make the same mistake now...)