Occasionally when developing a web application in Eclipse (3.2.1 as of writing) I see the following problem reported:

The validator XML Schema Validator is a delegating 
validator but no delegate can found for it.

This is caused because ValidatorDelegatesRegistry.getInstance() is not thread safe. Cleaning the porject causes the problem to be removed from the problem list.

This is a known bug (a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=155706">155706) and is being worked on.