The fossil web server uses the "Cache-Control: private" directive to avoid storing the data on intermediate proxies. It does not use other directives to keep the browser from caching the data. At least on the Opera browser this leads to the browser returning stale pages. According to RFC 2616 the private cache control allows the browser to cache the pages:

See the RFC:

private
Indicates that all or part of the response message is intended for a single 
user and MUST NOT be cached by a shared cache. This allows an origin server to state that the specified parts of the 
response are intended for only one user and are not a valid response for requests by other users. A private (non-shared) cache MAY cache the response. 

Therefore I suggest to use one of the usual methods like No-cache etc. maybe in addition to the private.

Without this change fossil is unusable from at least the Opera browser.