Not sure where to report this, but when you put an external link to Wikipedia using the graphical edit interface (not editing the source directly), like for example to https://en.wikipedia.org/wiki/Mani_stone, it will get converted to a wikibooks link, like this:
[[wikibooks:Mani_stone|Mani stones]]
That is obviously quite unexpected. What is more tricky, this only happens once you hit save - in a preview, it is displayed correctly.
I suspect there might be some empty wikibooks link in the wiki due to this.
Directly in the wiki text, when editing an article. I think it happened to me when I copied a different link and edited it by pasting a wikipedia link into it.