1. Open the huge file with a lot of tags and nested children. 2. Select the container which contains a plenty of childred (i.e. div, h:form, etc.) 3. Start to change its tag name by quick typing to smt like h:formabcdefhigklmnopq1234567rst... 4. With every new letter refresh/update will be performed. 5. Very soon you'll get this NPE.