Begin building a galactic wonder on a planet square.
Then change your opinion and delete that built (use the x at the top of the building list) to free up the space on that planet : the galactic wonder is gone forever.
- no planet can now build it
- if you save your game an reload, this problem remains
In complement, it seems odd that Trade Goods should take one space on a planet. The same can be said about wonders, which are costly and risky to build (some other civ may get them before you do) and should probably not incur a planet space loss ; why not build wonders/trade goods on "free" space ?