Edited:

Looks like this was OS X. In 10+ years with Solid State I've never seen this happen. Go figure.

Can I ask what went wrong?

Can I assume that everyone knows to always 'Lock' SD cards before they insert them into computers.

OSX, even old versions, happily read the native quicktime files from my LS300.
If you record in AVCHD then you'll need to import to an NLE first or playback the files in VLC for example.

Macs will add all sorts of invisible files if you aren't careful.

If you drop encoded files such as mp4s onto an SD card for playback in a TV and you are using a Mac then you should eject the card using an app likeHidden Cleaner. This will remove files like '.DS store' etc so that your non Mac devices don't get confused with the indexing copies that OSX will put on the SD card.