Wonder smonder I'm minting gold coins

There is a bug where another civ completes a wonder you are working on. If you then replace the build (click something else) the wonder moves to second in the list. Then next turn you will get the gold for shields message but the wonder remains in the production list!! If you finish the top item any overrun will go to the wonder, which then next turn will give you gold for shields. This doesn't seem to ever get removed unless you do so yourself (that is as long as its not the first in the list it will remain).

The gist is that you effectively get a 100% shields to gold build without currency and obviously two times better than building wealth anyway (since wealth only nets 50%).

This is even funnier when the wonder in question gets a production bonus for marble or stone.

I've seen this as well. I didn't notice the shields-to-gold thing (was in the middle of some heated action) but it was strange seeing that wonder stay on the list despite the fact that it was already built. I started wondering if I could maybe actually build the thing myself afterall. But it goes away if you let it queue back up to the top and complete a turn of production. So it's a minor exploit that requires a bit of micromanagement, but not a game killer. Could be handy in the early game now that I know about the gold thing. I'm not entirely sure how you reproduce this yet. That might take some experimentation. It probably has to do with the fact that a city will automatically (sometimes annoyingly) switch back to the most recently pre-produced item that has the most shields. So if you switched from that build, and it was built by somebody else, it will stubbornly stay on your city's production queue. Atleast I'm pretty sure that's how I did it. From your description, you might have done it slightly differently. Maybe you had it as part of a long queue?