It is possible to simple method to prevent a server from coming back a mistake 400 (bad request) whenever a URL is asked for that contains a percent % symbol?

Many major sites have this same issue, for instance: http://twitter.com/% or http://bing.com/%

But other sites have fixed this and return a custom error page rather, as opposed to the standard server response, for example: http://facebook.com/% or http://google.com/%

It is possible to simple, single method to escape just one character % symbol from coming back a 400 error, but rather, coming back a 404 error utilizing a custom error document using mod_rewrite or similar in Apache?

It is possible to simple, single method to escape just one character % symbol from coming back a 400 error

Yes, you need to urlencode any figures you devote a url, if they're not envisioned having a unique meaning.

For instance in PHP:

$string = urlencode('%');

FYI % is encoded as %25: https://twitter.com/%25