The original PlayStation did not have any writable memory, so you had to buy "memory cards" that were plugged into the front in order to save game progress.
Sony sold official cards. But since they only held 15 "blocks", and some games took up an entire memory card for one save file, there was a huge market for third-party memory cards (some much larger than 15 blocks). My question is focused on the 15-block ones.
When I bought my first memory card, I went for a third-party one to save money. It just looked a bit uglier visually, but that didn't matter to me in those days.
When I saved Final Fantasy VII, turned off my PlayStation, went to bed, and woke up the next morning, there was no save file. I repeated this procedure one more day before concluding that the memory card consistently "forgot" save files if the PS was turned off. I ended up buying an original Sony memory card and it still has my FF7 save file on it to this day.
Judging by reviews and comments from others at the time, this was common, which baffles me. As I understand it, these memory cards were not battery-powered, and could not be opened up, but had some sort of persistent flash memory in them.
How they could fail at keeping data like this, on a technical level? And even if others' cards didn't erase data every night, but "only" sometimes, what could be the reason for that?