Gangplank's barrels do not apply lich bane procs
I am assuming that this is a bug because the barrels apply Sheen and Trinity Force's spellblade procs, but not Lich Bane's, probably because gangplank isn't expected to do magic damage. Pictured in the attachments is the bug in action - Parrrley with lich bane doing more damage than Parrrley with lich bane on a barrel.