Warn with two copies of React

src/renderers/dom/client/ReactMount.js

@@ -795,6 +795,18 @@ var ReactMount = {
 
     var deepestAncestor = findDeepestCachedAncestor(targetID) || ancestorNode;
 
+    if (__DEV__) {
+      // This will throw on the next line; give an early warning
+      warning(
+        deepestAncestor != null,
+        'React can\'t find the root component node for data-reactid value ' +
+        '`%s`. If you\'re seeing this message, it probably means that ' +
+        'you\'ve loaded two copies of React on the page. At this time, only ' +
+        'a single copy of React can be loaded at a time.',
+        targetID
+      );
+    }
+
     firstChildren[0] = deepestAncestor.firstChild;
     firstChildren.length = 1;
 

src/renderers/dom/client/__tests__/ReactMount-test.js

@@ -199,4 +199,29 @@ describe('ReactMount', function() {
       element.unmount();
     });
   }
+
+  it.only('warns when using two copies of React before throwing', function() {
+    require('mock-modules').dumpCache();
+    var RD1 = require('ReactDOM');
+    require('mock-modules').dumpCache();
+    var RD2 = require('ReactDOM');
+
+    var X = React.createClass({
+      render: function() {
+        return <div />;
+      },
+    });
+
+    var container = document.createElement('div');
+    console.error = mocks.getMockFunction();
+    var component = RD1.render(<X />, container);
+    expect(console.error.mock.calls.length).toBe(0);
+
+    // This fails but logs a warning first
+    expect(function() {
+      RD2.findDOMNode(component);
+    }).toThrow();
+    expect(console.error.mock.calls.length).toBe(1);
+    expect(console.error.mock.calls[0][0]).toContain('two copies of React');
+  });
 });