This may not be specific to dashboards, but iv seen it happen twice in dashboards.
(I am at HEAD as of right now)
When viewing the dashboard I get the following exception:
AphrontDuplicateKeyQueryException: #1062: Duplicate entry 'PHID-VOID-00000000000000000000-builtin:project.png' for key 'originalPHID'
I have seen it under a recent tasks query, and an active diffusion repo query.