Report and screenshot here: https://www.mediawiki.org/w/index.php?title=Talk:Flow&workflow=rsw3ybqad0yu3fig

Partial copy:

If $1 is in the header of a flow post, such as "Parametr $1 translation message not found", a weird bug shows up in watchlists where $1 is replaced with the username block of the user

Version: unspecified
Severity: normal
URL: https://www.mediawiki.org/w/index.php?title=Talk:Flow&workflow=rsw3ybqad0yu3fig
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=65143