Strange behavior with sessions

I have two browsers open: in FF I'm logged as admin, in IE I'm logged as normal authenticated user.
As I perform an upload (and form submit) with SWFUpload in FF, I suddenly became admin in IE.
That doesn't happen the other way around: if uploading and submitting in IE I still remain admin in FF.
I don't know if it's an SWFUpload issue or not, but I'm reporting anyway.