The WORST example of this happening for me was when I had been going for one of those wonders (I think Chichen Itza or Notre Dame?) and my capital said I was 1 turn away from finishing. Well, that 1 turn passed, and it didn't finish. I thought it was strange, but didn't think too much of it until hit "Next Turn" again only to find that the AI finished it in some far off land!
I was really annoyed, so I loaded an autosave to investigate why it didn't finish on the turn that it was supposed to. After loading and reloading a few times, I figured out that it was because my Golden Age ended, and the expected completion time was counting on the +20%
, but apparently the Golden Age ends before the city adds the hammers into your production. Without that extra +20%, I fell 3 hammers short. Had I known that the hammers would get added in this way, I could've manually added 3 hammers for that turn. But the city screen told me it was 1 turn away, and I believed it.