When trying to add a JIRA Gadget to a Confluence page (see https://confluence.atlassian.com/display/DOC/Adding+JIRA+Gadgets+to+a+Confluence+Page) and that gadget has "url" as a parameter, the gadget will not be displayed.
Editing the macro, Confluence shows the error message, that the gadget with the URL that was specified as the plugin's "url" parameter was not registered with Confluence.
This was noticed in the Custom Content for JIRA plugin version 0.3 (https://marketplace.atlassian.com/plugins/com.tngtech.jira.plugins.customcontent.jira-gadget-custom-content). Renaming the parameter fixed the issue.