The @warn rule is designed just for that. It's written @warn <expression> and it prints the value of the expression (usually a string) for the user, along with a stack trace indicating how the current mixin or function was called. Unlike the @error rule, though, it doesn't stop Sass entirely.

{% codeExample 'warn' %} $known-prefixes: webkit, moz, ms, o;

@mixin prefix($property, $value, $prefixes) { @each $prefix in $prefixes { @if not index($known-prefixes, $prefix) { @warn "Unknown prefix #{$prefix}."; }

  -#{$prefix}-#{$property}: $value;
}
#{$property}: $value;

}

.tilt { // Oops, we typo'd "webkit" as "wekbit"! @include prefix(transform, rotate(15deg), wekbit ms); }

$known-prefixes: webkit, moz, ms, o

@mixin prefix($property, $value, $prefixes) @each $prefix in $prefixes @if not index($known-prefixes, $prefix) @warn "Unknown prefix #{$prefix}."

  -#{$prefix}-#{$property}: $value

#{$property}: $value

.tilt // Oops, we typo'd "webkit" as "wekbit"! @include prefix(transform, rotate(15deg), wekbit ms)

.tilt { -wekbit-transform: rotate(15deg); -ms-transform: rotate(15deg); transform: rotate(15deg); } {% endcodeExample %}

The exact format of the warning and stack trace varies from implementation to implementation. This is what it looks like in Dart Sass:

Warning: Unknown prefix wekbit.
    example.scss 6:7   prefix()
    example.scss 16:3  root stylesheet