How can the commanders signature be invalid?

if an AF command sponsorship letter was approved and signed by a MPF AF commander, how can the signature be invalid? The letter was unknowingly filled out incorrectly according to the AFI but the commander still approved and signed. should this be on the military member or should the fault be on MPF and/or should the member be punished for this action being misinformed? should the member be excused from any or all consequences from this mistake?