Original Sass and libsass can produce different output for functional pseudo-classes. Original Sass does not normalize whitespace contained within an expression, and it also accepts invalid expressions. Libsass is stricter, and removes superfluous whitespace. E.g., on the following selector:

nth-child( 2n  + 3  )

Original Sass produces:

nth-child(2n  + 3)

whereas libsass produces:

nth-child(2n+3)