Clicking a link versus using the address bar

[eluser]cassowary[/eluser]
Hello,
I am maintaining a CI application developed by a variety of people over the past two years.
Right now I am addressing a user requirement involving the authentication of a site visitor based on data in his URL. Sometimes this code works; sometimes it doesn't.

The most puzzling thing I'm dealing with is the different behaviour of the code when I click a link (i.e. an anchor to URL A) in a page and when I explicitly try to load the same page at URL A by placing this into the address bar of the browser. In the first instance, I'm able to invoke the authentication code; in the latter case, it seems to fall through. It's almost as if, when I use the address bar to load the page (and, one assumes, short-cuts in the browser favourites) I am invoking different code to when I click upon a link to the self-same page.

I'm sorry if that sounds garbled. Can anyone elucidate matters?

Cassowary