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.

$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);
}

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