ERROR: Ignored call to 'alert()'. The document is sandboxed, and the 'allow-modals' keyword is not set.
Recently I found this issue while writing code snippet in "JSFiddle". And after searching, found this was happening because of new feature added in "Chrome 46+". But at the same time Chrome doesn't have support for "allow-modals" property in "sandbox" attribute.
Chromium issue for above behavior:
To make it work you have to add "allow-scripts allow-modals" in "sandbox" attribute, and use "window.alert" instead of "alert".
Feature added: Block modal dialog inside a sandboxed iframe.
Feature working Demo page: