An apparatus and a corresponding method for processing a sequence of received data items are disclosed. The processing is performed by multiple processing elements. A reorder buffer comprising multiple slots is used to maintain the order of the received data items, wherein a processing element reserves a next available slot in the reorder buffer before beginning processing the next data item of the sequence of received data items. On completion of the processing a buffer change indicator value is read by the processing element when seeking to insert the processed data item into the reserved slot. If the buffer change indicator changes during the course of the insertion process, this serves as an indication to the processing element that another processing element is modifying the content of the reorder buffer in parallel. A check may be repeated for at least one subsequent already-processed data item, since this latter data item may have become ready to be retired from the reorder buffer.