If you can see this page, doesn't that mean that it exsists? Even if an entered URL has not been made on the server, it still exsists as a page you have entered into the address bar. Because you have entered the in page, you requested the page, and since the creator didn't make it, it links to a 404 page. It has been requested, so it may not be exsisting but it exsisted during that request.
It also exsists in our mind. But then, that brings us to the question: How do we remeber a "404" page? Our neurons are incredible at remebering things; it's like if you had 10 million libraries inside of your mind. And that's just scratching the surface of neurons.
Now, although that might seem like we can remeber a lot, we aren't perfect when it comes to memory. Short-term memory can only last for about 30 seconds, and long-term memory can hold memory for an indefinite amount of time- that is, when it catches memories. The less a memory stays in your active mind, the less you think about it, or the more the memory "fades." Memory fades can happen when our neurons overwrite that information, and hence why they may seem "foggy", or even, disappear.
But then, that leads us to the question: how long does one memory generally last?
This page doesn't exsist...