user5091: The BOLTs frequently mentioned "fail the channel" when something unexpected is received or happens. For instance, when receiving an update_fulfill_htlc message, you should fail the channel if the received channel ID isn't known.
At this point, you would have money stuck in a channel, so does this mean you should try to close unilaterally, cooperatively, either? Is "fail the channel" defined in the bolts somewhere?

user5091: The meaning is defined in BOLT #5 Failing a Channel.

5091: <p>The BOLTs frequently mentioned &quot;fail the channel&quot; when something unexpected is received or happens. For instance, when receiving an <a href="https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#removing-an-htlc-update_fulfill_htlc-update_fail_htlc-and-update_fail_malformed_htlc" rel="nofollow noreferrer">update_fulfill_htlc</a> message, you should fail the channel if the received channel ID isn't known.</p>
<p>At this point, you would have money stuck in a channel, so does this mean you should try to close unilaterally, cooperatively, either? Is &quot;fail the channel&quot; defined in the bolts somewhere?</p>
 5091: <p>The meaning is defined in <a href="https://github.com/lightningnetwork/lightning-rfc/blob/master/05-onchain.md#failing-a-channel" rel="nofollow noreferrer">BOLT #5 Failing a Channel</a>.</p>
