About two months ago I asked whether coach passengers should use the first class lavatory?
My own view is that passengers should be accommodated in their ticketed cabin first. But extenuating circumstances — a drink cart blocking the lavatory, a long line — make it ok to use the lavatory in a different cabin. Also that it’s usually ok to use the lavatory when the seat belt sign is on; what’s more important is what it smells like when you’re done.