In OnDemand with instances that have (at least) JIRA + Confluence, the macro browser sometimes fails to load with the error:

There has been an error loading the macro browser. Please try again or see your system administrator.

Unfortunately there is nothing logged that gives a clue as to why it fails. I strongly suspect that it's caused by a timeout when attempting to pull in JIRA's gadgets into the macro browser. This would explain why it is sporadic.

The macro browser should handle this more gracefully (e.g. load the built-in macros and then continue to fetch the external gadgets in the background) or at least log what failed.

Resolution:
For details on how this was resolved, please see this comment.