When submitting certain words in content, you receive a 403, 404, 406, 500 or "Page not found" error message.
This behaviour is most likely caused by the Apache module mod_security. Depending on its filter settings, phrases that trip the module include
lynx, perl, mother, select from, table, cc:, and many more.
Ask your host to tone down the mod_security settings.
Alternatively you can try to add the following to .htaccess:
# Turn off mod_security filtering.
This will turn off filtering by mod_security.
Not all hosts will allow this, so you may need to contact your hosting provider.