Since Opencart gives you 404's all over the place, it's nice to have customers not leave the site when they hit this brick wall.
I've found the best thing to do is put your products on the 404 page and lead customers back into the shop.
Ok, so how do we do this?
Glad you asked.
We're going to make a new Layout route first, so go to System > Design > Layouts
Then you're going to want to INSERT (which i've always wondered why it was named this, I'd rather it say add new).
From here, you should make the Layout name "404" so you know what the heck it is, then you're going to want to make the ROUTE
Save all that and you're half way there.
Now you've got a way of displaying things on the 404 page, go to Modules and find what you want to display.
I'm going to use the Featured Module for this example. Go into it, add a new module and select 404 as your layout. Then you can add all your products and they'll show like they would if it was anywhere else on the site.
How do you feel now?