When central notice banners on Meta are edited, this is often done by a huge variety of users with a very different JavaScript skill set. Often, these banners contain JavaScript.
The issue is that the JavaScript is occasionally invalid or causes runtime errors. These errors will sometimes prevent execution of site scripts, for example when they are thrown in $(document).ready() or other handlers that execute queued callbacks without wrapping each call into a try{} catch(e){} clause.
Moreover these kind of errors are hard to track because they are committed at Meta.

wikimedia commons discussion (permanent link)