I have noticed this odd behaviour on my iPad 2.
I was playing about with the app "Paper by FiftyThree" and saved a fairly blank page with just some scribbles into Camera Roll. To my surprise when I went to look at it in camera roll there was an old deleted photograph overlaid with the scribbles! If I repeat the "save" I get the same scribbles overlaid on another, random, deleted photo.
If I save a virgin blank page I get an untouched deleted photo.
Note : it only appears on the thumbnail view - if you click on the photo you just get the recently saved file, not the deleted photo. The 3 finger tap & slide to zoom in does work but the quality is thumbnail-poor, as you would expect.
I note that when saving ( Paper app calls it "sharing") the "Save Background" switch has to be "off" for this effect to occur.
It's as though by setting the background to be OFF that the saved sketch is transparent and somehow picks some random complete deleted photo to use as background. Maybe the thumbnails are created and stored in a separate location and are not deleted in the same way as the parent photo? Or maybe the app writers took some shortcuts?
I don't know if this only happens with this particular app, or Camera Roll and not the other albums, or if it happens on other iPads or iPhones.
I can't think of any hardware reason for this peculiarity - it must be software.
Maybe this will give some insight to people who would like to know just what goes on deep inside IOS5.