gulp-sass, watch stops when invalid property name

This answer has been appended to reflect recent changes to Gulp. I’ve retained the original response, for relevance to the OPs question. If you are using Gulp 2.x, skip to the second section

Original response, Gulp 1.x

You may change this default behavior by passing errLogToConsole: true as an option to the sass() method.

Your task might look something like this, right now:

gulp.task('sass', function () {
  gulp.src('./*.scss')
    .pipe(sass())
    .pipe(gulp.dest('./'));
});

Change the .pipe(sass()) line to include the errLogToConsole: true option:

.pipe(sass({errLogToConsole: true}))

This is what the task, with error logging, should look like:

gulp.task('sass', function () {
  gulp.src('./*.scss')
    .pipe(sass({errLogToConsole: true}))
    .pipe(gulp.dest('./'));
});

Errors output will now be inline, like so:

[gulp] [gulp-sass] source string:1: error: invalid top-level expression

You can read more about gulp-sass options and configuration, on nmpjs.org

Gulp 2.x

In Gulp 2.x errLogToConsole may no longer be used. Fortunately, gulp-sass has a method for handling errors. Use on('error', sass.logError):

gulp.task('sass', function () {
  gulp.src('./sass/**/*.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(gulp.dest('./css'));
});

If you need more fine-grained control, feel free to provide a callback function:

gulp.task('sass', function () {
  gulp.src('./sass/**/*.scss')
    .pipe(sass()
      .on('error', function (err) {
        sass.logError(err);
        this.emit('end');
      })
    )
    .pipe(gulp.dest('./css'));
});

This is a good thread to read if you need more information on process-control: https://github.com/gulpjs/gulp/issues/259#issuecomment-55098512