Yeah - what happens if you win World's Fair while in a golden age? I'm pretty sure the 500 points get added in the background. My last game, I had a few turns left in a golden age, and then I noticed it was saying I had 16 turns left (epic speed). I didn't notice it in time to see how it got extended - but it wasn't anything I did myself. I'm pretty sure the happiness counter triggered a new one in the background before the other one was over. (It was pretty amazing - I popped 2 Great Artists after it to extend it further; and I was getting double culture from the World's Fair, and popped 3 Great Writers... I ended up basically filling out the entire Rationalism tree in about 20 turns.)