Possible Clues:

• How you might have forgotten to have a periscope for this
• On view? There's some mistake
• Done with viewing, which could be a mistake
• Has one forgotten about the periscope for this?
• It may get forgotten that that's what the periscope provides
• Supervision — omission
• Show sadness about leading tycoon's initial mistake
• Mistake as voter failed to conceal expression of despair
• Inadvertent omission
• Exhale in public showing boob