Simple Acl controlled Application Tutorial & Cannot modify header information

I've just began to cook cakePHP. I am following Simple Acl controlled
Application Tutorial.
It's so frustrating that I'm having this error and don't know how to
trace it.

Right after creating app_controller.php, I started getting this error
on add group page. I finnished the steps anyway.
The group was created though. I tried to create a user:
AclNode::node() - Couldn't find Aro node identified by...

P.S: I double checked whitespaces in all the files I create or
modified twice.

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