Flashdata relationship to "Warning - Cannot modify header info" ???

[eluser]seanloving[/eluser]
I get this warning

Code:

Message: Cannot modify header information
- headers already sent by (output started at /application/libraries/MX_Model.php:1)
Filename: libraries/Session.php
Line Number: 662

I have isolated the error to one of my controllers where I make this call

Code:

$this->session->set_flashdata('location_id',2);

If I remove this call then no warning. Interesting.

I did the usual checking for whitespaces and I found none. Assuming no whitespaces, what do I look for next?

I understand one should not output to the screen until session has started. I'm now trying to understand what this means.

Anybody know of a connection between this Warning and $this->session->set_flashdata()???

Thanks
--Sean Loving