Wordpress - How to prevent automatic redirection of 404 errors and "incorrect" URLs?

This worked for me:

remove_action('template_redirect', 'redirect_canonical');

As Ash suggested, you can turn off the feature by using the following code:

remove_action('template_redirect', 'redirect_canonical');

In looking at the redirect_canonical function in canonical.php, it would appear you can also modify the behavior with your own filter.

At the end of the redirect_canonical() function, there is a call to filter the final answer:

$redirect_url = apply_filters( 'redirect_canonical', $redirect_url, $requested_url );

So you could write your own filter to modify the final redirection or return null to stop the redirection, based upon the input, thus turning off the feature for a particular URL or a subset of URLs.

You can disable permalink guessing for 404s without disabling redirection of canonical URLs by adding the following line somewhere in your code (eg. in functions.php):

add_filter('do_redirect_guess_404_permalink', '__return_false');

Relevant functions in the Wordpress code are redirect_canonical and redirect_guess_404_permalink in wp-includes/canonical.php.