Fail when test resolution method is overspecified

lib/runnable.js

@@ -181,7 +181,8 @@ Runnable.prototype.run = function(fn){
     , start = new Date
     , ctx = this.ctx
     , finished
-    , emitted;
+    , emitted
+    , result;
 
   // Some times the ctx exists but it is not runnable
   if (ctx && ctx.runnable) ctx.runnable(this);
@@ -213,7 +214,7 @@ Runnable.prototype.run = function(fn){
     this.resetTimeout();
 
     try {
-      this.fn.call(ctx, function(err){
+      result = 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]') {
@@ -224,6 +225,10 @@ Runnable.prototype.run = function(fn){
         }
         done();
       });
+
+      if (result && typeof result.then === 'function') {
+        return done(new Error('Asynchronous resolution method is overspecified. Specify a callback *or* return a Promise, not both.'));
+      }
     } catch (err) {
       done(err);
     }

test/acceptance/overspecified-async.js

@@ -0,0 +1,8 @@
+describe('overspecified asynchronous resolution method', function() {
+  it('should fail when multiple methods are used', function(done) {
+    setTimeout(done, 0);
+
+    // uncomment
+    // return { then: function() {} };
+  });
+});