The announcement banner allows the addition of html or javascript code, without any validation of this entries. Failure to close a tag can break the interface such that JIRA dashboard will go blank and it will not be rendered. That includes all the administrators functions, thus they will not be able to revert back and fix it.

The bug with the workaround was reported here

It will be nice if there was some html and javascript code-checker for the announcement banner to indicate that there is missing or not closed tags.