Strange issue with forms auth.

Hi all,
I made a web apllication with forms authentication active, everything worked fine one thing that I had was that if users select remind me it seems the day after they have to login again
if found this link http://weblogs.asp.net/scottgu/archive/2005/11/08/430011.aspx and changed the value of my forms timeout
<system.web>
    <authentication mode="Forms">
          <forms timeout="180"/>
    </authentication>
</system.web> 
that seems to work, but now users are saying me that if they visit the site somethimes they are logged in as another user without doing something, huh ???? How is that possible ?
Thanks for input
Filip
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