I write custom gadgets for my company and they always worked well in JIRA (adding them to Dashboard). Now we need to add them to our Confluence so we added an external gadget feed and the gadgets show up in macro browser. However, after clicking Insert button, they don't appear on the page at all. If added using wiki macro, they show up, but can not be edited and block the page from saving.
Sometimes it magically works though so this may be hard to reproduce. Please try more than one time the whole process.

I created a simplistic gadget for JIRA using the command atlas-create-jira-plugin, then I changed atlassian-plugin.xml to add the gadget, and created file gadget.xml. No other changes have been made! I attached the source codes (testGadget.zip) as well as the compiled jar file (testGadget-1.0-SNAPSHOT.jar).

Steps to reproduce:
1) Download the attached JAR file.
2) Install plugin in JIRA, add it to dashboard, see that it works.
3) Go to Confluence, add-ons, external gadgets and add feed from your JIRA.
4) You may need to set up trust connection between JIRA and Confluence.
5) In Confluence, create blank page, go to insert, other macros and in external content, pick our "TestGadget".
6) Try to add it.
7) Fail.

This is definitely a bug if only for the reason that it does not throw any kind of error, warning or anything else in logs or elsewhere.