<===> input.scss
$foo: 10%; // line 1
// line 2
$error: $foo + 20px; // line 3

<===> error
Error: 10% and 20px have incompatible units.
  ,
3 | $error: $foo + 20px; // line 3
  |         ^^^^^^^^^^^
  '
  input.scss 3:9  root stylesheet

<===> error-libsass
Error: Incompatible units: 'px' and '%'.
        on line 3 of input.scss
  Use --trace for backtrace.