Running a map_reduce operation with a JavaScript function can fail with the error "A single operation cannot use both JavaScript aggregation expressions and $where" even if the query doesn't contain a $where instruction. This can happen when the command is sent to a shard that is updating the target collection with new data. This operation leaves the internal state of the JsExecution partially configured, so that, when the operation is automatically retried, it looks like it has been created by a $where instruction