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)