In the blitz test, many users reported errors in the breadcrumb display when using the 'Known Location" tab

We have no good story for reproducing the exact problem but some situations that will break the breadcrumb display are when the space you have selected no longer exists or if the page no longer exists e.g. still in the index but no actually in the DB.

e.g. try these URLS:
https://node2.qa-eac.atlassian.com/pages/breadcrumb.action?spaceKey=CONFDEV&title=Home - will return JSON as expected
https://node2.qa-eac.atlassian.com/pages/breadcrumb.action?spaceKey=CONFDEV&title=Homeyyyy - will return HTML
https://node2.qa-eac.atlassian.com/pages/breadcrumb.action?spaceKey=CONFDEVIANT&title=Home - will return HTML

Trawling the logs for the blitz test show that some of the cases may have been related to the destination spaces have a custom theme configured which was no longer present -
e.g.

2009-11-04 22:53:45,353 WARN [TP-Processor83] [atlassian.confluence.themes.DefaultThemeManager] extractTheme Unable to find configured theme module: com.atlassian.confluence.themes.modern:modern
 -- url: /pages/breadcrumb.action | userName: tcampbell | referer: https://qa-eac.atlassian.com/pages/editpage.action?pageId=1783332921
2009-11-04 22:53:49,083 WARN [TP-Processor16] [atlassian.confluence.themes.DefaultThemeManager] extractTheme Unable to find configured theme module: com.atlassian.confluence.themes.modern:modern

Might be a red herring.