Now, it's a perfect 404 page (Status 404 with simple message body, the game is injected by JS, so it's not registered by any crawlers.) For a better understanding, things that might not be clear for any direct-linked visitors:
Most of the site's content is about classic computing in one way or the other. Also a main field of activity is in interactive web content (including a tradition in porting classic arcade games to JS). So it did make sense to offer an interactive error page, which is perfectly in tune with the main theme of the site. (So it's a perfect check for any item on the check list in the link referred to above.) Also it was meant as a humorous comment on interactivity, as you may either watch the interfering error being destructed to ruins (where the invaders would represent the site and the tank the user) or you could get even hands-on by "reaching trough" the error and interacting with the site (by shooting at it for being so dumb and lazy and not providing the right document).
Also, the error page isn't exactly new, in fact it is as of March 2012. (Which is still quite fresh, considering that the design of the main pages is as of 1999. Yes, I'm a bit lazy with this. But at least there's a current error page.)
[Edit] Just for info: An extra set of touch controls is injected, if a touch-enabled device is detected (supporting webkit/blink-style events or the ms-pointer-API as used by IE10). – For the benefit of SmartPhone- and tablet-users.