Pressing 'BACK' in browser and not getting previous page

[Astara]

Sometimes, when I press back, I can't get the previous page to display -- something about the operation not being allowed
and that the page must be regenerated.

Why does this happen?

The reason I ask, is I came across the HTML spec on the w3.org site, url https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html,
specifically section 13.13 on History Lists.

User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity retrieved earlier in a session.

History mechanisms and caches are different. In particular history mechanisms SHOULD NOT try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the user saw at the time when the resource was retrieved.

By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism SHOULD display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents.

This section seems to say that I shouldn't be disallowed from pressing "back" and seeing the previous page. So what overrides section 13.13 -- as it seems to say that the history mechanism is not for semantic accuracy, but for user convenience. Shouldn't that prevent a situation from where pressing 'back' doesn't present you the previous page?

Thanks for clarifying and a great x64 port.
-A*a...

[Tomaso]

In some cases, this "issue" is simply caused by redirection.
When clicking the Back button, you go to a page that instantly redirects you to the page that you were trying to leave.
Instead of clicking on the Back button, try holding it down for half a second..
This will give you a history of where you've been, and lets you choose exactly where to go back to.

[adesh]

It does not happen for all the pages in history, only for the pages where you arrive after submitting some data (generally using HTTP POST method). For such cases it is a safeguarding mechanism to avoid potential mishappenings, e.g. duplicate transaction on a bank website.

[Moonchild]

Another case where this happens is when the server explicitly indicates in the response headers that the page should not be cached.