With the RPX plugin you are supposed to get redirected to the page where you pressed "Log in" once you have successfully logged in. Most often this is also true. However, if you try this, click Log In, on the log in page click Register, now "change your mind" click Log In again. This time log in with your preferred OpenID provider. Then, on successful log in, you will be redirected back to the register page - not the page where you originally clicked Log In.
This is not the optimal behavior (IMHO).
Note: This only works on sites where you can register of course.