Example of how Sass mixins when used excessively and without thought can actually be more of a code smell than a helper.

Maintainability.md

We had multiple mixins for prefixing properties such as:

• border-box
• border-radius
• box-shadow
• ...on and on...

When realistically we just needed...

@mixin vendor ($property, $value) {
    -webkit-#{$property}: $value;
       -moz-#{$property}: $value;
        -ms-#{$property}: $value;
         -o-#{$property}: $value;
            #{$property}: $value;
}

...which you would use as for example: @include vendor(box-sizing, border-box);

Yes this adds prefixes that might not be needed (e.g. border-radius is reasonably well supported in older versions of Firefox and WebKit) but this repeatable content will get chewed up when it comes to GZIP'ing.

Of course this doesn't help when prefixed properties have different value structures or when a value needs to be prefixed. You can use Compass to adress some of these issues, but for bulletproof support my preference is post processing with Autoprefixer.