This behaviour is reprehensible and I have no respect for anyone who tries it on.
Noticing Malaga Mike's post, I once voluntarily gave up 1C for a gentleman who had developed breathing problems and took his seat in economy for the remaining 90 mins of the flight - seemed the right thing to do, as there was more space for the crew to administer O2 etc.
Quite often, I'll offer to swap row 1 with elderly or disabled pax in other rows in business, as the extra legroom makes them a little more comfortable.
But try to get into a cabin I hadn't paid for, no.