Tomcat: Manager Loading Issues

Here's another 'good samaritan', 'hopefully this'll help someone down the road' post. I recently put a clean Tomcat 6.0.18 instance on my machine, and deployed my app. Everything worked fine with the app, but when I tried to access the manager application, I wasn't prompted for the login credentials like expected. I figured it might have been an issue with my (customized) tomcat-users.xml file, but it wasn't - when I checked out the manager log file, I saw a bunch of references to this error:

javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.DocumentBuilderFactoryImpl not found

I was just getting a 401 page with the message "authorization is required" or something to that effect. I did some digging, and found a bunch of halfway answers that included reinstalling Tomcat - this isn't really acceptable, so I figured I'd look in tomcat/lib, and sure enough there was no xml jar there, so I copied in xerces-impl-xxxx.jar in there, restarted, and could access manager no problem - not sure why this happened, but if you ever see that, make sure you have a xerces parser in your classpath...