@Jaime said:
Using the query string is ugly and can get baked into bookmarks and using Session variables gets all weird when people do "Open in new tab/window".
Also, saved tabs can cause big problems. I used a web contact form to contact a foreign graphics design company, after I was done with sending the message I left the page (showing a 'thanks for contacting us' message) on a tab in case I got no email response and needed to do it again.
Every day after that I got an email from a different rep at the company thanking me for my enquiry, I thought this was very weird after a couple of days especially as they had already started my work...but just assumed their whole sales department was drunk or something.
Eventually after a week I got a 'PLEASE TO BE NOT SENDING US ANY MORE QUOTATION REQUESTS!!!!' message from them.
After a bit of investigation, and vigorously denying abusing their contact form, I found it was being caused that little lonely tab I had forgotten about. Every morning when I started up Firefox it submitted the same damn message. Nice design there...