Miro Posting before Migo

Hi,

I have come across cases where the user has posted the miro on posting date 31.March.2010 (Entry Date 6th April 2010) whereas the MIGO for goods / Service Entry for the PO has been done with posting date 5th April 2010 (Entry Date 5th April 2010)i e Miro is getting posted prior to MIGO. The PO has all the requisite check box ticked such as GR based invoice / SE based invoice.

Is this a Standard SAP functionality or can we put a validation to ensure user does not make the mistake.

While on the subject is it possible to reverse a migo for which miro has been done without first reversing the MIRO.

Please advice

Mukesh