MessageTag.getBundle() uses findAncestorWithClass() to get hold of a BundleTag. But as a 
MessageTag does not have a parent-tag, null is returned. getBundle() proceeds and fetches the 
bundle out of the pageContext, if possible.

Normally, this is no problem, but consider this 
situation:

file1.jsp:
<i18n:bundle baseName="bundle.one" 
scope="session"/>

file2.jsp, participating in that session:
<i18n:bundle 
baseName="bundle.two"/>
<i18n:message key="key.from.bundle.two"/>

The MessageTag 
here cannot find bundle.two, because
a) getBundle() returns bundle.one from the 
pageContext
b) bundle.two was not placed there by BundleTag because bundle.one was already 
there (see BundleTag.doEndTag())

so we get a NullPointerException.