I fitted 3 old dears 2 mortice locks and 2 cylinder locks last week, they called me back 30 mins later and said the locks have stopped working,
I couldnt quite understand why but had an inkling, i turned up at the door and yes i was right, i told them to try the other keys in the other locks and they worked

Poor dears had got them mixed up

IM ONLY JOKING of course i didnt charge them, i even gave them some differant coloured keyrings so they could tell the differance in future.
I always find the elderly need another 2 or 3 keys cutting for each lock for neibours and family.