What's the biggest s/charge bill you've ever been landed with?

And what was it for? (Other details such as size of block, location and year works happened [to put cost into perspective] etc helpful).

Horror stories invited, as I'm trying to buy a BTL which has been served with a s.20 notice, only it is such early stages that there is zero indication of what works/how much.

I've looked broadly at what might need doing to the block and mentally costed it, but thought it might be helpful to hear some worst case scenarios...things I'd never imagined...