add a note about side effects in created/beforeCreate

Author: yyx990803

en/universal.md

@@ -12,6 +12,8 @@ Because the actual rendering process needs to be deterministic, we will also be
 
 Since there are no dynamic updates, of all the lifecycle hooks, only `beforeCreate` and `created` will be called during SSR. This means any code inside other lifecycle hooks such as `beforeMount` or `mounted` will only be executed on the client.
 
+Another thing to note is that you should avoid code that produces global side effects in `beforeCreate` and `created`, for example setting up timers with `setInterval`. In client-side only code we may setup a timer and then tear it down in `beforeDestroy` or `destroyed`. However, because the destroy hooks will not be called during SSR, the timers will stay around forever. To avoid this, move your side-effect code into `beforeMount` or `mounted` instead.
+
 ## Access to Platform-Specific APIs
 
 Universal code cannot assume access to platform-specific APIs, so if your code directly uses browser-only globals like `window` or `document`, they will throw errors when executed in Node.js, and vice-versa.