On a specific project I encountered some issues with encoding.
When I used save_and_open_page the html page opens with bad encoding chars ( accent chars ).
To fix this I just add on my layout header this forgotten meta :
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
During some tests I encountered this warning randomly due to faker data.
This is due to regexp apply on utf8 string with /n modifier. Modifier is used to overridden with encoding source, and generate this warn.
To fix this I remove the /nmodifier.
Source : http://ruby-doc.org/core-2.2.2/Regexp.html#class-Regexp-label-Encoding