Better callback misuse detection

EventEmitterEx.js

@@ -138,13 +138,16 @@
             }
 
             function callback (position, err/* arguments */) {
+                assert(! Array.isArray(result[position]),
+                    'Callback called more than once by function at position ' + position + ' (0-based)');
+
                 if (err) {
                     firstError = firstError || err;
+                    result[position] = [];
                 } else {
                     result[position] = slice(arguments, 2);
                 }
                 len--;
-                assert(len >= 0, 'Callback called more than once for each mapAsync() function!');
                 if (! len) {
                     if (firstError) {
                         eex.emit('error', firstError);

test/EventEmitterEx.test.js

@@ -354,7 +354,19 @@
                     cb(null);
                     expect(function () {
                         cb(null);
-                    }).to.throw(Error, 'Callback called more than once for each mapAsync() function!');
+                    }).to.throw(Error, 'Callback called more than once by function at position 0 (0-based)');
+                    done();
+                });
+                mapped.on('error', done);
+                emitter.emit('end');
+            });
+
+            it('should throw if callback called too many times (with misbehaving callback)', function (done) {
+                var mapped = emitter.mapAsync(function () {/* bad function, does not call cb() */}, function (cb) {
+                    cb(null);
+                    expect(function () {
+                        cb(null);
+                    }).to.throw(Error, 'Callback called more than once by function at position 1 (0-based)');
                     done();
                 });
                 mapped.on('error', done);