I am on Windows 7 SP1.
I do know it seems to happen mostly on pages where I would add a comment. I remember that because when it happened I got frustrated since it wouldn't submit the comment and I would have to copy what I wrote to a Notepad or something. Refresh the page then copy it back. But I didn't pay close enough attention on whether it happened on other pages too.
It could happen at anytime. But again, I didn't pay close enough attention but will from now on.
If I notice anything else I will post it.