Custom 404/error page
This code will send users to a 404 page that you provide rather than a random 404 page your host provides. Creating the 404 page is as simple as creating a page in muse and calling it 404. This code cannot redirect to an absolute URL which means that you cannot put your domain into this code (e.g. http://example.com/404.html). The URL must be a relative URL (e.g. 404.html or /404.html).
Example
# Example of valid markup
ErrorDocument 404 /404.html
# or
# ErrorDocument 404 /
# This will redirect 404/error pages to the home page.
# This might seem appealing but Google does not recommend it.