ORA-19948: piece does not contain a cross-platform datafile backup, reason=20

I received this error when running an RMAN recovery of a cross-platform incremental "from scn ..." backupset:

ORA-19948: piece does not contain a cross-platform datafile backup, reason=20

No matter how many times I repeated my incremental backup I received the same error.  I was surprised to find no hits at all anywhere, other than generic references to the error code documentation, so I wanted to record here what I found.  In my case a datafile had been added to the source database after my initial level 0 backup, and so the problem with my incremental "from scn ..." backupset was not that I didn't use the "from platform ..." clause, as the error code documentation suggests, but that the backupset included a datafile that was not yet