The LogPageNotFound plugin for MODX Revolution logs all requests that result in a 404 (page not found) error. You will almost certainly find that there are links out there for pages you've changed the alias of and possibly links at your own site that no longer work. Using the information in the log, you can create either weblinks on the site or redirect rules in your .htaccess file to send visitors to the right place.
When examining the log, you'll also see evildoers looking for a variety of security holes trying to access files and directories that never existed. Once you know what they're looking for, you can block them in .htaccess.
The plugin does not slow down your site, because it only executes when a page is not found.
Tutorial at:
http://bobsguides.com/logpagenotfound-tutorial.html.
(Would somebody please install this guy at modx.com -- there are many important links to modxcms.com that no long work.)
