Idoc Error Workflow - Question

Hi whenever an Inbound Order Idoc comes and creates an Order and if it fails in 51 status we receive the error in the Inbox according to the partner profile settings.

At one occurence we received the workflow message for a successfully posted Idoc with the success message in it(Order saved with Order number in it).I verified the Idoc its in 53 status and it never went to 51 status...Can any one explain the possible reasons for this..?

Task used- Standard Task - TS 91100001

Any help is deeply appreciated.Thanks.

Larry.