With @import, it's possible for a file to refer to a mixin or function that gets redefined later on. Right now References will just override the previous definition and treat it as though it always referred to whatever the last value was, but the more correct behavior would be to throw an error, since changing the definition isn't supported in the module system.

For example:

<==> input/entrypoint.scss
@import "definition1";
@import "upstream";
@import "midstream";

<==> input/definition1.scss
@function fn() {@return 1}

<==> input/definition2.scss
@function fn() {@return 2}

<==> input/midstream.scss
@import "definition2";
@import "upstream";

<==> input/upstream.scss
a {b: fn()}

I don't think this is worth fixing before we launch, though.