Browsers send all "matching" cookies to a server when they connect.
A cookie consists of:
Name: the name of the cookie
Data: the stuff in the cookie
Domain: the domain name of the website to send the cookie to
Path: the path to match on the server[typically /]
Ignore path for the moment, domain matches are the domain of the host being connected to and anything up the chain.
So for http://gary.users.sherodesigns.com
Any cookie for gary.users.sherodesigns.com; users.sherodesigns.com; sherodesigns.com; and .com would be sent. Browsers don't allow cookies to be set for top level domains[.com] because of this. If multiple cookies with the same name match, then they are all sent. So for magento you have a your logon cookie as follows:
The staging server will receive:
PHP grabs one of the 2 cookies at what might as well be random, so things work, then they break, then they work, then they break..
A common host setup for us is:
www.client.com --> client.com
client.com --- main website
dev.client.com -- dev site
staging.client.com -- staging site
So if you are doing anything on both the main site and dev or staging, cookies being set by the main site can interfere.
I do not have a GOOD answer to this problem. A quick hack is to copy:
To local and edit line 44 and make it unique for dev, staging, and live:
const SESSION_NAMESPACE = 'adminhtml';
In PHP 5.3 it is not possible to use an expression when setting a constant. so
const SESSION_NAMESPACE = $_SERVER['SERVER_NAME'].'adminhtml';
Won't work. It should work for PHP 5.6 but I don't recall at what point this was changed.