Example of nasty deep nesting from original Polygon styles.

Ohgodthenesting.scss

#nav_social {
  float: right;
  margin-right: 50px;

  & > ul {
    position: relative;
    border: none;

    & > li {
      display: inline-block;
      padding: 0 10px;
      font-size: 16px;

      &.fb {

        .fb-like span, .fb-like iframe {
          min-height: 20px;
          min-width: 74px;
        }

        & > a {

          &:before {
            background: url(/images/polygon/sprites/icons.png) no-repeat -7px -108px;
            content: "";
            display: inline-block;
            width: 22px;
            height: 36px;
            position: relative;
            top: 10px;

            @media
            only screen and (-webkit-min-device-pixel-ratio: 2),
            only screen and (   min--moz-device-pixel-ratio: 2),
            only screen and (     -o-min-device-pixel-ratio: 2/1) {
              background-image: url(/images/polygon/sprites/icons@2x.png);
              background-size: 108px 1043px;
            }
          }
        }
      }
    }
  }