I got that error several times last night on my Win7 64-bit PC running 1.8 RC3. My repeatable procedure was:
- Open a FIT Lum image file
- clone image by dragging upper left edge "tag" and dropping in workspace
- operate on image (various operations, undo, redo, with and without previews)
- save the cloned image to disk with a new name
- close the original FIT image file
and got the error every time. It didn't seem to cause any subsequent grief, but it looked like a serious enough error that it might be a problem later (orphaned pointers, or whatever).
Just another data point for you.
Rod