Initiating a session may overwrite your own custom cache control header, which may break clicking back to get back to a prior post request (on Chrome at least).
On my system it was setting 'no-store', which is much more severe than 'no-cache' and what was breaking the back-button.
If you are controlling your own cache headers carefully you need to call:
session_cache_limiter('');
...to stop it changing your cache control headers.