Special Redirect Code

Please don't do that -- any time you have a site behavior that tricks the client and behaves in a random manner, you're creating a design that destroys human trust.  My reaction to such a behavior would be to report it as a potential attack site to Google and never come back.

There may be better ways to accomplish the objective, perhaps via use of an API.  Can you tell us a little more about what you're trying to do - just in plain-language non-technical terms.  If we recognize the design pattern or if you can show us other sites that do something like this, perhaps we can suggest a better approach.

Depending on what you are trying to do... it sounds like you are thinking about it backwards.
Rather than randomize the click behavior, change the href for x% of page hits.
e.g., when the page loads, run a function that updates the link's href for x% of the users.

If you are trying to do a "20% of clicks are winners, click to find out" (and you don't the link's href to give away whether they are a winner or a loser), the link should go to a single page which uses server-side code to determine if the user is a winner or loser and either redirects to the appropriate page or loads the appropriate content.

^^ Scott -- good suggestion!