When I create a psd in CS6, and then open it in CS4, I get this error:
This document contains unknown data which will be discarded to keep layers editable. To preserve the original appearance instead, choose Flatten to load composite data as a flattened image.
And then I have the options to Flatten or Keep Layers. If I Keep Layers, things seem OK, but I'm wondering what data is being lost. I'm asking this question because I'd like to begin working in CS6, but I want to make sure anything I do in CS6 will still be editable in CS4 in case I don't buy CS6 right away when the beta trial period is over.
That isn't an error, just a warning that means what it says.
And you'll get that warning whenever you use new features in a document and try to open it in an older version.
If you use features new to CS6, they may not be editable in CS4, and won't always have the same appearance - hence the warning.