Occassionaly I run accross this error in PHP, how can I fix this or what cause it?
Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively. in Unknown on line 0
Just disable session.bug_compat_42 and session.bug_compat_warn as the error message suggests.
You are relying on a deprecated feature of PHP, called register_globals. This feature caused many security issues, and shouldn't be used any longer. The PHP manual discusses this in-depth.
For an excellent description of the problem, see this Google Groups post.
Edit: If you aren't relying on register_globals, then you should pay attention to Gumbo and stereointeractive's answers.
If your variable names are the same as the session parameters then this version of PHP will incorrectly recognize this as the programmer incorrectly relying on register_globals for session variables. Rename your variables and the warning should go away.
$mySessionVar = $_session["sessionVar"];
and not
$sessionVar = $_session["sessionVar"];
I would NOT suggest you disable the warnings or error messages, this is a problem you probably want to fix. Either you have variables names that are triggering bogus error message (see sterrointerative.com's answer) or you're trying to use global variables without defining them (which means you likely have bugs you can't see). In my opinion that's an important difference you want to sort out.
If it's only happening on some pages, you're likely going to have to review all the code pulled into those pages and search for use of variables that don't appear to be defined before they are used. Stepping through those pages with a debugger ought to help.