1. render a scene with F12 then exit image editor with ESC
2. modify an object in your scene
3. go to image editor with F11 and press "J" to swap render slot (example from "slot 1" to "slot 2")
4. re-render your modified scene with F12 then swap *3-4 times* beetween the two render slots with "J" key
5. now return to your scene (ESC) and try to undo (Ctrl+Z) object modification you did previously...

...you'll see you have to repeatedly press Ctrl+Z for every J pression done in the image editor until you can undo that very object modification.
Another side effect of this is you completely loose your second render in the slot 2 because if the history delete even the first J pression... blender assumes you never started the second render! :)

No .blend attached because every file is good to testing, anyway you'd have to follow those steps to have at least some action in your history to undo.

System specs:
Linux 64 bit
Nvidia card with GeForce 260.19.36
Blender SVN 35424 compiled with scons and Blender 2.56a 64 bit official Beta

Thanks a lot for your hard work!