Symptoms
There are reseller's and provider store configured on the same domain but different subdomains. If both of the stores are opened in Internet Explorer, there are problems with the correct stores work - for example, it is impossible to login, no error occurs, but the customer is not logged in. In other browsers the same configuration works fine.
Cause
The issue is Internet Explorer (IE) specific, the reason is a way IE works with cookies for subdomains, see details below (Q3):
http://blogs.msdn.com/b/ieinternals/archive/2009/08/20/wininet-ie-cookie-internals-faq.aspx
Workarounds
You can set www prefix for the stores (e.g. configure redirect store.example.com -> www.store.example.com) or use different domains.