I have seen a few times so far. I am looking through a micechatters trip report pics and surprisingly find myself in the background!
I think it might be interesting to see others experiences here in this thread!
The most recent time this has happened was at the Star Tours re-opening. We happened to be there at rope drop and a guy posted a pic that was taken about 10 or 12 feet behind us as we approached the Star Tours entrance.
Another time, we were in the park on a Sunday morning, riding the horse drawn trolley towards the main gate. The "In The Parks" photographer caught a shot up main street with the trolley in the foreground while we were in the trolley!