Why is an exception thrown if any output has occurred in calling JSP, when forward action is encountered ?

Question

Forward action must occur prior to any HTML template data or scriplets or expressions that write data to OutputStream. Why so?

Answer 1

It can be caused by a classpath contamination. Check that you /WEB-INF/lib doesn't contain something like jsp-api-*.jar
If it does, manually deleting the jar file solved the problem