Each Neocities website is given a not_found.html page by default. You can customise this if you want. However, here's a tip:
If you place any image on this page you can't just refer to it as: any_image.jpg
If you do so, and that image is in the parent directory along with the not_found.html file, that image will only load for 'missing' pages that are sought after in the parent directory. If a visitor tries to access a 'missing' page from another directory the image will fail to load.
I discovered this when I added Brian the Robot to my not_found page:
You must refer to the image as https://[your_site].neocities.org/[any_image].jpg
i.e. giving its full address on the internet.
What else to add to a
not_found page to get a
visitor back on track? "Please
click Back on your browser"?
"Please contact the