Update pitfalls.ms (#2226)

@Inject must preceed @observer

docs/best/pitfalls.md

@@ -58,6 +58,27 @@ For more info see [what will MobX react to?](https://mobx.js.org/best/react.html
 `@observer` only enhances the component you are decorating, not the components used inside it.
 So usually all your components should be decorated. Don't worry, this is not inefficient, in contrast, more `observer` components make rendering more efficient.
 
+### `@inject('store')` before `@observer` will cause MobX to not trigger 
+
+The effect with React is that the it will never render on observable changes.
+
+This is wrong
+```typescript
+@observer
+@inject('store')
+```
+It must be 
+```typescript
+@inject('store')
+@observer
+```
+
+You'll notice a warning
+```
+Mobx observer: You are trying to use 'observer' on a component that already has 'inject'. Please apply 'observer' before applying 'inject'
+```
+
+
 ### Don't copy observables properties and store them locally
 
 Observer components only track data that is accessed _during_ the render method. A common mistake is that data plucked of from an observable property and stored will for that reason not be tracked: