If a user stumbles across it and it just shows the default "sorry, page not found" message, it can often leave the user very confused, and the default message is not very helpful at all.

You want to customize it specifically for your site, and include a way for your users to find their way around your site, and maybe even a search box to find what they were looking for.

I like to share the ways to improve E-commerce site usability..

Identify users with their e-mail address.
Break up the ordering process into bite size chunks.
Tell users where they are and where they're going.
Don't make the ordering process harder than it needs to be.
Address common user queries.
Highlight required fields.
Make the ordering process flexible.
Put users' minds at ease.
Have users confirm their order before buying then provide confirmation.
Send a confirmation e-mail.