Enter b:en: in the search box of https://gan.wikipedia.org/, it goes to https://incubator.wikimedia.org/wiki/Wb/gan?goto=mainpage instead of https://en.wikibooks.org/.

Enter en:b:, it goes to the right site.

The intermediate target gan.wikibooks.org doesn't exist and will be redirected to incubator. In such case the interwiki link goes to a wrong place.