Using the current 1.0.5 packaged version of the code the BatchWriter returned by Table.batch_writer() will raise exceptions if there are any unprocessed items returned from a batch_write.

The cause appears to be the client re-encrypting the already encrypted unprocessed items that get returned, and in the process validating the attribute names against the reserved attributes which fails.

Replicating this is easy enough, use a client to put items into a table at a rate that causes the writes to be throttled. Once at least one item fails and gets returned as an unprocessed item it'll trigger that error.