A VC10K4 once returned with 'suspected fumes' in the back when the rear cabin filled with mist.
But they weren't fumes at all. It had been a damp, soggy day and when the cabin pressurisation was fully enabled on climb out, the mass of air in the back rapidly cooled - and a nice cloud of strato-FunBus formed in the back until the air had circulated!
This used to be mentioned in ground school, but as time went by it had been forgotten. The captain in question said he'd never heard of it and tried to blame everyone else for not telling him....
I had smoke on the flight deck a few times - and the odd smoke detector alert. But never both at the same time!
All of which were duly IR'd!