Comment 1 for bug 1105077

This is actually explained in the documentation:

       Replicas using row-based replication
           pt-table-checksum requires statement-based replication, and it sets
           "binlog_format=STATEMENT" on the master, but due to a MySQL
           limitation replicas do not honor this change. Therefore, checksums
           will not replicate past any replicas using row-based replication
           that are masters for further replicas.

           The tool automatically checks the "binlog_format" on all servers.
           See "--[no]check-binlog-format" .

So the error is expected if the slaves have MIXED format. I was going to mark this ticket as Not A Bug, but since this isn't the first time this message has caused confusion, we're obviously conveying things wrong, so I'll tag this for 2.1.9 and 2.2 and reword the error.