Whenever a joining cluster node tries to join a cluster and its setup to do so via SST it will fail due to an issue when calling the mktemp helper program.
Older versions of mktemp allowed trailing characters after the string of XXXXXX for the template. Version 7.4 does not and exits with an error of "mktemp: too few X's in template `wsrep_sst_xtrabackupXXXX.pid'" if you try. Newer versions have a --suffix option to address this, but that is not an option on a default ubuntu 10.04 system.
We're fixing the issue manually by changing line 118 in wsrep_sst_xtrabackup from "XTRABACKUP_PID=$(mktemp --tmpdir wsrep_sst_xtrabackupXXXX.pid)" to just "XTRABACKUP_PID=$(mktemp --tmpdir wsrep_sst_xtrabackupXXXX)"