I restored a RedHat VirtualBox VM from a colleague. I installed the VirtualBox extensions, changed some files on the box and rebooted. Everything went back. Tried again. Never changed. Made a snapshot. No difference.
After a little googling, found this article. Could the file system be read-only?
mount -o remount,rw /VolumeXXXXX /
It sticks. Something in the VM preparation changed the filesystem. Weird.