Well, that was unexpected.
Damp, disintegrating flooring on the other side of the wall - yes. Trail of black mould on this side of the wall, nearly breaching the other wall into our Garage - very much NO.
And the smell!
Well, once they stripped off the hardwood flooring and that plastic membrane stuff we got our nostrils filled with that smell of damp. Horrid acrid damp. This is us only at day 1 of a 2 week demo, refit and re-finish our entire downstairs.
Thankfully, by the end of day one's demolition all the bad chipboard, flooring joists and hard wood flooring was outside. Electric heaters deployed to dry out the wall.