In [7]: g.put('testing 1 2')
...
OperationFailure: command SON([('filemd5', ObjectId('4c755c247b98305a8b000000')), ('root', u'fs')]) failed: db assertion failure

In [10]: conn.test.error()
Out[10]:

{u'code': 13092, u'err': u'GridFS chunks collection can only be sharded on files_id', u'n': 0, u'ok': 1.0}

Having the error code and string would make this easier to debug.