Extends can be very bad.

avoid-drunkextends-output.css

/*
Using silent class
*/
.Button, .Random {
  background: red;
}

div .Button {
  color: red;
}

/* VS */
/*
Using a non silent class
*/
.Button2, .Random2 {
  background: blue;
}

div .Button2, div .Random2 {
  color: red;
}

/* Note that the css output on the last class is doubled.*/
/*
Alternate solution - use an attribute selector when you nest to stop the extend.
*/
.Button3, .Random3 {
  background: yellow;
}

div [class*="Button3"] {
  color: orange;
}

avoid-drunkextends.scss

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// ----

/*
Using silent class
*/

%background {
  background:red;
}

.Button {
  @extend %background;  
}

.Random {
  @extend %background;
}

div {
  .Button {
    color:red;
  }
}

/* VS */


/*
Using a non silent class
*/

.Button2 {
  background:blue;  
}

.Random2 {
  @extend .Button2;
}

div {
  .Button2 {
      color:red;
  }
}

/* Note that the css output on the last class is doubled.*/

/*
Alternate solution - use an attribute selector when you nest to stop the extend.
*/

.Button3 {
  background:yellow;
}

.Random3 {
  @extend .Button3;
}

div {
  [class*="Button3"] {
    color:orange;
  }
}