SSIS, MSDTC, and SQL Server clusters

Note to self: when receiving the following error from an SSIS package, the first thing to check is the Authentication setting for MSDTC.

Description: The SSIS Runtime has failed to enlist the OLE DB connection in a distributed transaction with error 0x8004D00E "The transaction has already been implicitly or explicitly committed or aborted".

The security setting should be set to No Authentication Required. Mutual does not appear to work with clusters. This is especially significant if, like me, you have a non-clustered instance in QA and a clustered one in Prod and you have been beating your head against the wall trying to figure out why MSDTC, with all the same settings, works in the former but not the latter.