Can't exec "xtrabackup_51": No such file or directory at /usr/local/
#### fatal error: no 'mysqld' group in MySQL options
Absolutely freaking beautiful.
Turns out that it's caused due to the automatic yum update to version 2.1.x of the xtrabackup application, which uses the MySQL 5.1+ plugins and fortunately, it's a very quick fix, even though I nearly pulled out what little hair I have to figure it out. Thankfully percona includes the last version with their repo, so it's just to specify the version and it's gonna work again.
0. Got root/sudo
1. Remove existing xtrabackup package.
yum remove xtrabackup
2. Check that the older version of the package is available from the repo
yum list | grep percona
3. Specify/Install version 2.0 explicitly (choose based on your OS version!)
yum install percona-xtrabackup-20.x86_64
It's as simple as that! No need to pull out your hair just yet or try to build from source, which is it's own piece of work thanks to cryptic documentation.
To avoid this from happening again, I'd suggest disabling the percona repo when doing a yum update for the entire server, or manually patching the version by calling yum update percona-xtrabackup-20.