We upgraded a Rails 2 application to Rails 3.2 and Ruby 2.1, changed the mysql adapter from mysql to mysql2, but did not activitate the asset pipeline. Instead we used Sass the old-school way (stylesheets in public/sass/*.sass) and relied on stylesheet_link_tag to activate the Sass compiler.

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Now all Sass-generated stylesheets inserted the following text into body:before:

Encoding::CompatibilityError: incompatible character encodings: UTF-8 and ASCII-8BIT

I could get rid of this by removing all generated .css files in public and clearing the tmp/sass-cache directory.

When this didn't help, try do add these following two lines above YourAppName::Application.initialize! in your environment.rb:

Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8

Note that this error can also be caused by other things during an upgrade from Ruby 1.8.7 to something higher. See Google.