Why would [super init] ever return nil, when "super" is NSObject?

Possible Duplicate:
In Objective-C why should I check if self = [super init] is not nil?

In Objective-C book i am reading, it is said that when [init] message is sent to NSObject, on occasion it may return nil and we should check the return value, before sending more messages to what may end up being a nil.

self = [super init];

if (self) {
 do stuff
}

I ask you though, what needs to happen for an NSObject to not be able to init itself?

Edit: Question specifically deals with an instance where YourClass:NSObject.