Since we already know that Activities can be torn down and created again when the configuration of the device changes, it basically means that the onCreate() method of the Activity can be called multiples times while not changing (moving away) the Activity itself.
So, consider that there is a call like this in the onCreate() block:
Observable.interval(0, 5, TimeUnit.SECONDS)
It will be executed multiple times. If this isn't cleaned up properly, it will most likely leak the Activity along with its allocated memory and the Observable itself, which will keep running in the background and consuming resources.
More experienced readers will have noted this already, but we have exactly this kind of leak in ...