i have problem with session

[eluser]minhbu[/eluser]
i use session save page previous before add new product.But when i save or add new product.My session is changed but i don`t know why?

Example

Page previous
http://localhost/web/product/list

i use $this->session->set_userdata("url",$this->uri->uri_string());
I save product/list
but when i click to http://localhost/web/product/add and save product.When i redirect previous.I get url session that have value is images/thickbox/loadingAnimation.gif.
I want to get product/list ?
I view source but i cant`t find images/thickbox/loadingAnimation.gif

I use native session.
Pls help me.Thank you.