This is not a bug, it is a security measure we use to protect the site. Do not report blackholed requests.
Forms on Blend Swap are valid only for half an hour, if your submission takes longer than this it will fail and you may get a 404 error.
This will also happen if you leave a form unattended for more than 30 minutes (say, the comment form) and then submit it.
This will also happen if you try to register and have cookies disabled in your browser.
If you get the 404 error: go back in your browser and RELOAD the page by hitting
CTRL + R or
F5 so you get a new, valid form.