To reproduce:
Look at your console.
Get test files from public/testImages.

Test 1: object unusable
Use the "large" files first.
Drop the test files on the file input below.
When the delay input is empty, some files should fail with the message "An attempt was made to use an object that is not, or is no longer, usable".
If none fail, try again. The error appears fairly frequently (at least on my machine).
Now put a number in the delay. 100 should be enough.
The error doesn't appear anymore. If it still appears, increase the number and try again.

Test 2: silent error
This one is much harder to reproduce.
Take the "small" files from the testImages folder.
Clear the delay input.
Drop all files on the input.
Sometimes, the counter does not reach the expected number, but there also is no error.
If everything worked fine, either try again with the same images, or increase the amount of images by copy-pasting.
But even with 200 images, it's hard to reproduce.

-> These bugs probably have to do with files being copied to workers simultaneously