Photoshop :: Error Message When Opening CS6 Files In Earlier Versions

PSCS6 with 12.0.4 update.  Mac OSX 10.8.2   16GB 1333 Mhz DDR3  543 GB Available
 
I've recently become aware that other people are getting an error message when opening many of my my psd files if they are on earlier versions of PS and also on PSE, (including the latest version).  The message is:
 
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. 
 
Three options follow:  Flatten  Cancel  Keep Layers
 
When 'keep layers' is selected the file opens as normal.  However, people with much earlier version (CS version 8) don't get the option to keep layers, and the layers are flattened when the document is opened.
 
I still have CS5 on my mac, and I find that if I take my finished file into CS5 and save it there, the customer does not get the message above, and the file behaves as it should.  Those on very early version of PS can also open the files without any issues. 
 
I've been using CS6 since it was released and have had to delve through all my files and save them in CS5.  Not every file is affected and I've been unable to find a common thread.