Magnetic storage + strong permanent magnets = bad. I’ve never really tempted fate on that, but I happened to be disposing of an old MiniDV camcorder, I had a couple of tapes, and I saw a chance to get the straight truth.
That is an arrangement that I would’ve considered blasphemous; I would be very worried for that tape.
That stack of magnets is an inch in diameter, N40 neodymium. Under the laminate that’s a sheet steel desk, and it’s just barely possible to get enough grip with a bare hand to pry the stack straight off, to give an idea of how strong they are.
What I found is that a situation like that is not that bad. The tape will be fine.
Minimum serious damage is done by placing the magnet directly on the tape:
This will leave the video mostly intact. There will be corruption every second or two (once per revolution of the spool maybe?) which travels in a vertical wave left-to-right over the video frame, likely due to the damage being greater on the side of the tape nearest the magnet combined with the slanted i.e. helical tracks. Even though this is digital data it still uses the clever spinning tilted head design.
It turned out to be much harder to destroy tape than I’ve always assumed.
Swiping the magnet across the surface of the cartridge a few times is not sufficient, either. Video will be pretty much garbage, but nowhere near securely destroyed; there may be a few totally intact frames.
To really destroy the video on that tape with that magnet, it’s necessary to rub it down for a few seconds at least in a few directions and circles, like a polishing motion. After that the camera doesn’t recognize any data.
Video is far more forgiving than arbitrary organized data as on floppy disks and data tapes, and I don’t know exactly how much error handling data is present in this video format. I do know for sure that floppy disks are much easier to ruin. My advice going into 2015 is to keep magnets away from all of your floppies and cassettes. But still. Video tape isn’t such a delicate flower after all.