Why is Page_Load not firing after coming back from another page using ASP.NET - ergo epic embarrassment :)

The reason for the page executing doesn't affect the page cycle, the Load event always fires when the page is executed.

So, if the Page_Load doesn't run sometimes, it's because the page is cached and doesn't execute on the server. The page can be cached in the browser, in a router somewhere along the way, or on the server using server side page caching.

If you haven't enabled server side page caching for the page, it's cached in the browser or in the network. You can use cache settings to try to elliminate this:

Response.Cache.SetCacheability(HttpCacheability.NoCache);

This will keep the page from being cached in normal circumstances. (Check also that your browser isn't in offline mode, then it will use anything in the cache regardless of it's cacheability settings.)

When you navigate to a page using the Back button, the page is reloaded from memory, and no request is sent to the server.

You can confirm this using Fiddler.

I'm not sure if this is true in all browsers.

If you are redirecting, it's possible the client is caching the response. In order to get past this you might add an extra query parameter that simply holds the time.

This is usually enough to get past most pages caching mechanisms.