I got the following Nagios error after setting up replication between two MySQL 5.6 servers and monitioring the slave.
"UNKNOWN: should never reach this part"
This plugin was preivously working on MySQL 5.5 servers.
It appears that the MySQL 5.6 replication has a different output from MySQL 5.5. Therefore you will need to download the latest copy of check_mysql_slavestatus which fixes this issue.