Yes I have!
I had known my friend for years, we met at a local college and she lived on the same street as me.
She'd often pop round mine or we'd go out into our local area shopping, or having coffee or lunch, or doing things together.
It was years before I suddenly realised, hey she never invited me to her flat. She had an older teenage son who was being difficult and a random on / off lodger, so I put it down to that.
We'd meet at her gate on the street.
But one day she said to me 'don't ever come round my flat unexpectedly will you?' which made me wonder what's going on.
Anyway after many many years of knowing her, a situation arose whereby she gave me her spare keys and asked me to do something for her urgently.
Well I got the shock of my life, her flat was absolutely filthy from one end to the other and it was filled with litter all over the floors and stank. I won't say more but there was a lot more to it than that. I'm talking 20+ years of squalor and zero cleaning whatsoever plus infestations and 'issues'. (Whereas my mother had been a hoarder but a clean and organised one)
Weird thing is she was always immaculately dressed and turned out, she was also quite grandiose and snobbish now and then - the sort that would pedantically correct your poor use of English or make disparaging comments about others. So the idea she was living like that wasn't one of my suspicions.