If you have a form with multiple pages. I have buttons to navigate back to a previous page, but if I happen to be on the second page of a form and hit the back button in Internet Explorer, I'm taken to the page I was at before I even started the form.
This is not uncommon with many web applications. The usual browser navigation buttons are 'external' to the form itself and will do this.
Other users in the past have covered this by adding in an explanation to the first page of their forms to point out to users to navigate form pages using the controls on the form.
Where you have already used the 'page button' to 'go to next page' as I'm sure you have, also add a page button set to 'go to previous page' for users to go back to other pages.
If over all your forms you use a similar 'layout' where perhaps your 'go back' buttons are always in a similar location (the bottom left of a page is the common location) all your users will easily follow this for all other forms as well.
Also though, you could present a special link to your users to access the form.
You could for instance present the link to the form within an HTML link where you can supress the usual browser navigation buttons..
something like:
Fill in our form
but of course set the width/height to your form dimensions
