session var not changed if file not found error

This has baffled me all day on my FC6 php-5.1.6 based server. On a normal working page, I set a session variable at the top and another session variable in the middle of the page/script. This page has no errors nor missing links. So everything works great. Now, if I cause at least 1 image in the page to become missing on the server (ie, by deleting a .jpg file), then Apache logs the error as usual, ie: "File does not exist". But in addition, my two session variables seem to get deleted. If I put the picture(s) back, then no apache errors, and my session vars are set fine. I've even enabled detailed php error reporting and I get no errors. Is this normal session behavior? Please say no. Any ideas? I can't understand why missing links would cause a session variable's annihilation. -eric wood