Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.0.rc.1)
// ----

$screen-sm: 		30em;
$screen-md: 		48em;
$screen-lg: 		60em;

%fl {
  float: left;
}

.main {
  @media (min-width: $screen-sm) {
    background-image: url(img/nice-img.png);
  }
  
  @media (min-width: $screen-md) {
    width: 60%;
    @extend %fl;
  }
}

.sidebar {
  @media (min-width: $screen-md) {
    width: 40%;
    @extend %fl;
  }
}

SassMeister-output.css

You may not @extend an outer selector from within @media.
You may only @extend selectors within the same directive.
From "@extend %fl" on line 16.