I had a Seagate 8TB external HDD, and it was getting full so got another. They look identical and I've had them sitting side by side most of the evening. I was using my MacBook and realised I had to format the new drive to exFAT and so being extremely careless and overzealous to use my new drive I forgot check that I definitely had the correct drive in before formatting...
I am extremely fortunate that I have things backed up on various drives, especially sentimental things like old family videos.
I have lost some stuff though. The biggest thing being 2TB+ worth of PS3 & 360 ROMs. That was an unbearable process to collect and organise so I'm not looking forward to doing it again.
So yeah, I'm normally pretty good with my data but this was a big slip up. Moral of the story is be careful & back up your data on multiple drives.