I can reproduce this reliably on a new workspace with a new Grails project using Groovy/Grails Tool Suite 3.0.0.RELEASE (Build Id: 201208091018),
Andrew, do you know if your fix got in that build?
Steps to reproduce:
1) on a grails project, show the local history for a Groovy file (if there is no history yet, make some edits, save and try again)
2) in the history view, right click one of the entries and select "Compare Current with Local" (make sure you didn't have the compare editor open for that file)