Users are rarely, but at rates enough to be significant, getting sent to the FailPage for dead sessions. There seem to be two cases where this happens.
One, the user actually does come back with no session data, so we don't know who they are or what they were doing. Not much data logged here, but it has happened on two different browsers.
Two, the user makes a successful donation and the donor data gets cleared out of the session, then it appears we try to send another request to paypal, but since we've cleared the donor data, it looks to us like a dead session, even though they do have an active session, just without donor data.