Ban mixing of promise and async APIs in it handler.

lib/runnable.js

@@ -227,7 +227,7 @@ Runnable.prototype.run = function(fn){
     this.resetTimeout();
 
     try {
-      this.fn.call(ctx, function(err){
+      var ret = this.fn.call(ctx, function(err){
         if (err instanceof Error || toString.call(err) === "[object Error]") return done(err);
         if (null != err) {
           if (Object.prototype.toString.call(err) === '[object Object]') {
@@ -238,6 +238,9 @@ Runnable.prototype.run = function(fn){
         }
         done();
       });
+      if (ret && typeof ret.then === 'function') {
+        done(new Error('runnable accepted "done" but returned promise'));
+      }
     } catch (err) {
       done(utils.getError(err));
     }

test/runnable.js

@@ -406,5 +406,18 @@ describe('Runnable(title, fn)', function(){
         test.run(done);
       })
     })
+
+    describe('when fn had a done argument and returns a promise', function(){
+      it('should consider this an error', function(done){
+        var test = new Runnable('foo', function(done){
+          return { then: function(){} };
+        });
+
+        test.run(function(err) {
+          err.should.be.ok;
+          done();
+        });
+      })
+    })
   })
 })