This incident has been resolved.
Jan 11, 14:26 PST
A fix has been implemented. You should refresh your page before trying to delete a targeting rule.
Jan 11, 13:00 PST
It's worth noting that you can update/delete rules via the API. The issue is only via the web console.
Jan 11, 12:05 PST
We are investigating an issue where individual targeting rules cannot be deleted. In most cases, you can work around this via Deleting all targeting rules (which resets the split) and/or inheriting from another environment. You can also leave the targeting rule and update to have the desired impact. If you have any questions about a workaround for your situation please contact firstname.lastname@example.org
Jan 11, 11:57 PST