sourceMappingURL is wrongly relative to scss file, should be relative to css file instead.

$ bin/sassc --version
sassc 0.12.3 (sass/libsass 3.4.3)
$ mkdir -p scss target/css
$ touch scss/style.scss
$ sassc -g scss/style.scss target/css/style.css
$ cat target/css/style.css
/*# sourceMappingURL=../target/css/style.css.map */

If browser tries to access ../target/css/style.css.map from target/css/style.css, it would get 404 on target/target/css/style.css.map.

Doesn't seem like C libsass's issue.