Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v0.7.0)
// ----

%extend {
  @media only screen and (max-width: 600px) {
    width:100%;  
  }
}

.extend {
  @extend %extend;
}

SassMeister-output.css

@media only screen and (max-width: 600px) {
  %extend {
    width: 100%; } }

Here the libsass processing diverges from the Sass 3.3.5 processing by failing to pass the extended selector down into the media query.