Adobe Photoshop CS3 Extended error with documents

Hi there,
This is not the first time when my projects in Photoshop show up errors. After this error my project looks different with lost data. Can anyone help me to skip these errors ? Why I have this errors and can this be prevented ?
Thank you !

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.
Keep Layers Cancel Flatten
