Web to Case bug

my last post did not submit properly.

When you create a web to lead form you are asked to enter a result page or thank you page etc.

When I use the web to lead form 1/2 the time it correctly sends me to the nominated thank you page as nominated when the form was created.

The rest of the time it incorrectly takes me to my CRM home page , logged in and the case is not created. If i use the back buton and re submit the correct thank you page is shown and the case is now created

please advise