I published tables with FILESTREAM data before, but it seems like there is a particular planetary alignment that triggers an error during the execution of the snapshot agent.
This unlikely combination consists in a merge article with a FILESTREAM column and two UNIQUE indexes on the ROWGUIDCOL column. Yes, I know that generally it does not make sense to have two indexes on the same column, but this happened to be one of the cases where it did, so we had a CLUSTERED PRIMARY KEY on the uniqueidentifier column decorated with the ROWGUIDCOL attribute and, on top, one more NONCLUSTERED UNIQUE index on the same column, backed by a UNIQUE constraint.
Setting up the publication does not throw any error, but generating the initial snapshot for the publication does:
This is a rather specific confluence of events, so it probably won’t affect many people. Still, it is a bug.