I thought I was well enough used to them - the usual sort of thing - my mother utterly convinced her sister had stolen their mother's house, my fil telling us the housekeeper he didn't have was a prostitute. Night after night he'd found her in bed with a man. Realized later that after a stay with his other son he'd imagined their house was his own, his dil who cooked and cleaned was the housekeeper, and of course while wandering about at night he'd found the brazen hussy in bed with a man - his own son.
Took some working out, though.
However, the most fantastic (and distressing) was probably after I'd made the mistake of watching Who Do You Think You Are? with my mother. She'd apparently been enjoying it (as far as she could enjoy any TV) but the person in question had run some ancestor to earth in Venezuela, only to find that they'd been put in a pauper's grave with no headstone.
In the middle of the night I found my mother up and terribly distressed. She was utterly convinced that she and her cleaning lady had taken my father's dead body (he'd died at least 12 years previously) in the cleaning lady's car to a graveyard 100 miles away, where her parents were buried. And there they had simply dumped my father's body.
I knew at once that she'd had a bad dream triggered by WDYTYA. In vain I told her over and over that I'd been there when my father died, I'd been there when the undertakers took him away, I'd been there at the funeral. None of it made a scrap of difference. In the end I phoned the cleaning lady - could she just tell my mother whether she'd ever taken her anywhere in her car? (I knew she hadn't - didn't mention the body!)
Even when she spelt it out, my mother was still convinced. It wouldn't have mattered except that she was so terribly distressed about it.
No matter what anybody said, it was a good 48 hours before the conviction started to fade, and even then she insisted that someone had come to the door and asked her to get rid of a dead body! I did actually say, come on, maybe that might just happen if you were a gangster in a rough area, but who is honestly going to come to your door - a total stranger - and ask an old lady of 89 to get rid of a dead body?
'Well, I don't know, but they did!'