Cisco Switch Interface Showing (err-disabled)

What to do if a cisco switch port shows down/down and err-disabled?

For example:

#show interface fastEthernet 1/0/8
FastEthernet1/0/8 is down, line protocol is down (err-disabled)

From Cisco's error decoder:
<blockquote>
%ETHCNTR-3-LOOP_BACK_DETECTED: Interface [chars]: [chars]
Explanation: The problem occurs because the keepalive packet is looped
back to the port that sent the keepalive. Keepalives are sent on the
Catalyst
switches in order to prevent loops in the network. Keepalives are
enabled by
default on all interfaces. You see this problem on the device that
detects and
breaks the loop, but not on the device that causes the loop.
Recommended Action: Issue the no keepalive interface command in order to
disable keepalives. A disablement of the keepalive prevents
errdisablement of
the interface, but it does not remove the loop.
NOTE: In Cisco IOS Software Release 12.2(x) SE-based releases and later,
keepalives are not sent on fiber and uplink interfaces by default.</
blockquote>

To fix add 'no keepalive' to the port's configuration.