Adding observer to NSNotificationCenter second time causes EXC_BAD_ACCESS

Question

Hypothetical scenario:

In my viewDidLoad method I am adding view controller as an observer for custom notification (say, notification MyFooNotification). Later in the process when view is loaded the notification gets posted and controller processes it. When I leave the controller with it's view I DO NOT remove the observer (intentionally) in viewDidUnload. Next time when opening the view, the observer gets added again, but now when the observed notification gets posted - I get EXC_BAD_ACCESS.

Can anyone explain why this is happenning.

P.S. I do know that I should remove it in viewDidUnload I'm just curious about the lower level details.