Error message when opening CS6 files in earlier versions

Question

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.
Hoping someone can throw some light on this.

Answer 1

This similar to the message one should get in an older version of photoshop such as photoshop 7.
If one presses ok instead of read composite that might preserve the layers with the original look.
But sometimes if a layer blending mode or some other feature is used an earlier versions of photoshop doesn't have, reading the composite is the only way to preserve the look of the image.
You said your using shape layers?
If they are shape layers you didn't use a stroke from the tool options bar?
Can you post an example of the image with the layers panel visible that you get that message with?
I guess your using photoshop cs6 with the 13.01 update?