Possible Clues:

• Inadvertence
• You've forgotten what the periscope is for
• A lapse in supervision
• Shot giver due to unintentional omission
• To express desire in public is a mistake
• It's a mistake to give South Home Rule -- or could be
• How one's forgotten to use a periscope for this
• What gets forgotten when you get the periscope
• Supervision - failure to notice
• The sort of mistake that 14 down might make