CodeIgniter - Editing Output Buffer before sent to browser?

I'm researching the PHP framework CodeIgniter. I need some help regarding editing the output before it's flushed to the user.

Usually in PHP, you can just use ob_start(); and then ob_get_clean(); to retrieve the contents.

While reading the CodeIgniter documentation, though, they say all output is already buffered.

Has anybody experience with unintentionally starting a second ob_start() ? Alternatively, can anyone talk about the CodeIgniter output buffering so I may edit it?