If XA DML is submitted to an operating Galera cluster and innodb_support_xa is on, the cluster will crash as soon as an XA transaction is committed, and it must be rebuilt as a new cluster. If innodb_support_xa is off, the cluster won't crash, but XA transactions committed are not propagated to the other nodes, which puts them out of sync, causing the nodes to fail later when a contradictory change is posted.

I understand that XA is documented as not supported in a Galera cluster. The purpose of this issue is to have such transactions rejected when attempted, instead of causing the cluster to fail. (And, of course, it would be great if XA were supported in Galera, but that's not a bug...)