I don't think it's a mistake. It's probably a joke that references the "!1 phenomenon" meme. "In many cases, a string of exclamation marks followed by '1' can be read as a careless typo, though it has been increasingly used with intention to express a sense of urgency or excitement. While the configuration of QWERTY keyboard layout varies across different language zones, an exclamation point is almost always invoked by holding down the shift key and pressing the digit / symbol key (1/!). As a result, the erroneous input of '1' after a string of exclamation points became a common typo in online chat, especially when typing in haste."
On a related note, it's strange to see that Google's 404 error pages don't include useful information: a search box and a list of relevant pages. For example, when you type in the address bar www.google.com/chat, Google should suggest www.google.com/talk or this page about Gmail Chat.
Google could even use Chrome's "Oops!" pages. "Google Chrome can show suggestions for the page you were trying to reach, if it is unable to connect to the web address. Instead of a cryptic error message, you'll see suggestions to navigate to other parts of the website or to search for the page with Google."