v3.3.10 / Arch Linux
Note: this is not a version-specific bug. I have had it on multiple versions in the past.
libtorrent: 188.8.131.52 / Qt: 5.7.1
Repeated "I/O Error" notifications.
No "I/O Error" messages.
Okay, so I've had this bug a few times and I always thought it was related to the amount of torrents I had running, but I'm pretty certain I've discovered the true cause.
That's it. Do that and you should start getting flooded with notifications before long about I/O errors. That said, I haven't tested this much so it might only happen with certain torrents for unknown reasons.
If you can't reproduce the bug by unchecking a file, try unchecking a folder instead.
To get the error messages to go away, all one has to do is manually delete the partially downloaded file/folder and they will stop.
It seems like this bug would be fairly easy to fix by simply telling qBittorrent to remove unchecked files from the hard drive, but I don't think that would be the best solution since if somebody accidentally unchecked a very large file it would mean re-downloading hours or possibly even days worth of data (depending how well-seeded the torrent in question is).