Customised 404 error pages for web sites are a sensible idea. You arguably don't need a customised 418 page, but that didn't stop Google creating one — complete with an animated teapot.
As Google Operating System explains, 418 was a joke error code introduced by the IETF. The requirement?
Any attempt to brew coffee with a teapot should result in the error code '418 I'm a teapot'. The resulting entity body MAY be short and stout.
Click the teapot and watch it pour, then back to your day jobs. If you need to waste more time, check out more IT Pro Easter eggs.