Many web applications have limitations that are not disclosed to users. If you happen to use an application too frequently or upload too much data, your access will be disabled. In most cases, you'll see a message that informs you about the penalty, but Google Notebook chose a different strategy: showing a 404 error message.
"Back on December 20, I wrote about my frustration with Google Notebook, which I use every day as a way to archive much of what I read online (...). For some off reason, it seemed like Google Notebook was down, and yet I could find no other description of the issue online, which made me think I must be mad," writes Ran Barton.
He received a message from Google that explained the problem. "Your account was accidentally blacklisted by a blacklisting heuristic that looked at total size of notebook information. We've revised the heuristic, so you shouldn't be accidentally blacklisted again in the future."