We sometimes have to move Epics to a different project. After doing so with the new Epic implementation in place (Greenhopper 6.1.x) we have noticed, that Epics still show their corresponding stories in the "Issues in Epic" list, but on the SCRUM boards they show up with:

Issues
Total: 0
Completed: 0
Unestimated: 0

Story Points
Total: 0

A Re-index can solves this until the next Epic is moved.