Tomcat kept throwing annoying NoClassDefFoundErrors at me although the class was in a jar that resided in webapp classpath (a jar in mywebapp/WEB-INF/lib). Also, it didn’t happen all the time, just “often”. Restarting the server sometimes helped, sometimes not. Testing the same webapp on the Resin app server gave no such errors.
To my (so far) great positive surprise, an upgrade from Apache Tomcat 6.0.10 to 6.0.14 seems to have remedied it. Excellent! It’s amazing how such basic things can make you smile 
I also found this such error 10 hours ago, while i am trying to move my application from tomcat 4.0.x to tomcat 6.0.x
Very interesting… as always! Cheers from -Switzerland-.