I've never had it happened to any of my PS1/PS2 memory cards, but I've seen it happen to my friends files. I think the reason they had their stuff cleared was because they had a 3rd party memory card, it was like one of those ones where you could store a lot more memory blocks then normal for the PS1, and one time it got completely formatted lol.

I've heard of it happening also with the PS3s, it actually deleted a save of mine once while I was playing persona 3, but the only time it corrupted was when I was trying to save, so I just saved again and it didn't go crazy the second time. It never deleted anything else so idk.