Threading Error : Assertion failed

There was a customer who got this crashed with this error message

It’s an app which I’ve modified to include multi-threading. I couldn’t recreate the problem locally and it seems like there is nothing out of the ordinary which the user is doing. Basically the process crashed when running two threads both of which are resizing some jpeg files, there is no overlapping of files.

Any idea what does this error means ?

I’m getting the same error lately, did you ever learn anything helpful?