User suddenly logged in as another user I face new problem in my site, we have reports from users that when they are surfing our site with their account, suddenly logged in as another user and continue surfing site as it.

I searched all modules for below patterns to find where global $user changed by mistake but found nothing.

$user-&gt;uid = 
$user = 
-&gt;uid = 


Is there any suggestion about this issue?
I am sure that there is developer mistake here, we have something like  you guested,a global $user information  overwrite by developer code,I suggest you found when this happened,when a comment submit?when a specified form submit and .... ? 
I think in custom module your developer first  write global $user; and in continue define $user variable,I suggest you search for global $user then look at afterwards codes.