Using the visual editor, create a new external link and add a description containing the character ] (such as "a [b] c"). When saving the page, the generated wikitext is [http://link a [b] c], which of course means the link ends after 'b'.
This is a classic wikitext issue, the usual workaroud is to use ] instead of ].
Change 344632 had a related patch set uploaded (by Arlolra):
[mediawiki/services/parsoid@master] T133267: Escape extlink content when containing ] anywhere
Change 344632 merged by jenkins-bot:
[mediawiki/services/parsoid@master] T133267: Escape extlink content when containing ] anywhere
Mentioned in SAL (#wikimedia-operations) [2017-03-27T20:29:26Z] <arlolra> Updated Parsoid to 6eaad376 (T160599, T161178, T133267)