Magento: ?___SID=U appearing in some urls

Question

Can someone please explain why ?___SID=U is appearing in some Magento URLs on my site and not others?

I think it has something to do with sessions but I am not entir

Answer 1

Just something i come across today and though i make a comment, maybe it will help someone. I found that Magento will format/create an incorrect product url (inc. Session Id) if there were some errors during processing the page. This is not consistent though. It worth having a look at your server logs for PHP errors.