On the 1.9rc3 (remember it happen less) with me (another user warn me) it happen twice while i was "creating a new wiki page.
I was using french language.
I have set new menu item or create a link (i can't remember) on a page ( (tchat) ) (without the paces) and when clicking it i got a message this page is not created. Then i click on " Do you want to create it ?" Then it happen.
... I have read the Jeremy message, but it doesn't help me as the only cookie i knew have been eaten a long time ago !