What is happening if portlet blows up??

If you get a massive error message eg: null pointer exception....blah blah

this means that the portlet definition field is different in some way to the data source definition.

Eg:

Filter Field Application in the data source referred to token APPLICATION, but then realised it should have been APPLICATION_CODE. Changed this but did not go to the portlet definition and confirm the change there, therefore dashboard blew up