Involuntary downgrade

Being moved to a lower cabin than ticketed — an equipment swap shrinking the business cabin is the classic cause. You are owed the fare difference, calculated on rules that airlines apply with widely varying grace (the EU mandates a percentage of the ticket price; elsewhere it is the fare difference plus negotiation). Two practical defences: itineraries built on aircraft with stable cabin fits, and an agent watching equipment changes who can move you before the airport sorts it badly.