This was shortly after I tried flying after a battery swap without restarting the app and RC, which I don't usually do.  After rebooting software and RC, I was able to fly - I started flying and hit the "record" button, but nothing happened.  When I got back to my PC, one of my video files was corrupt.  I cannot copy it to my PC, and it won't play from the memory stick (VLC error "Bad file descriptor.")

Anyone ever encounter anything like that?

Had you powered off the drone first when changing the battery? Please don't forget this step.
And for the corrupted files, please try the following steps:
1. Plug the SD card back to the drone;
2. Power on the drone;
3. The files will be recovered itself if there are not too many corrupted files. And please check on DJI GO 4 app to see if the files had recovered;
4. If steps above doesn't work, and there are some important files you want to recover, please export the important corrupted files and one normal file and upload to Dropbox, I'll try to see if I can help;
5. You can format the SD card if there are no important files and you can film again.

I faced this and saw many users asking about this...
I'm really wondering how :

- this "dangerous" behaviour is not fixed already ( as it seems it also exists on other DJI models for a while ! )
- the user manual contains nothing about turning off recording before turning Off drone...
- the answer on how to solve this is not in a F.A.Q.

If DJI Thor method is not working (hint : it did not for me) you'll find a (working and free) fixing utility here :

http://djifix.live555.com/

(website is Off sometime)
The second solution which is never proposed by DJI has been working for me all the times.

Did the trick for me!

Any chance you forgot to stop video cspturing, before changing the batterie?
That was it in my case: unreadable file, no chance to open or to copy it.
Batt back in, switching on for some time.
I did even the copy with VLC

For me it was a normal shut down.  I shut down the Spark normally, but left the controller and app running.  Then, upon the Spark starting up again, I got the usual arbitrary flight ceiling issue and could not record video/shoot stills.  Then noticed the most recent video was corrupted.  It wasn't an important enough video to try and save, I just wiped the SD card and moved on.