It seems that people is using the [![whatever]] trick in Wiki 1.x pages to allow some filters using wiki-like syntax to work. Parser in 1.x transforms those double square brackets with exclamation marks into normal double square brackets so, later, filters are able to catch and process them.
To maintain BC, it's expected that all Wiki 2.x (nwiki) parsers respect this behaviour, so anything like:
will be converted to