Updating an issue with multiple uploads (attachments POSTed to /uploads.xml) in a very short time will likely (1) result in only one of them being accepted. The rest will fail with 422 Unprocessable Entity response and no error information in the response body (empty body).

I'm not sure how hard would it be to fix this, but I think documenting (2)(3) the limitation is sufficient.

In order to mitigate the limitation we synchronized our attachment creation operation so that they are no longer created in parallel but are executed sequentially.

This is probably hard to reproduce with the web UI therefore the category is "REST API".

(1) in our case, in the test we implemented, trying to upload 5 attachments at the same time was enough to make it fail reliably
(2) https://www.redmine.org/projects/redmine/wiki/Rest_Attachments
(3) https://www.redmine.org/projects/redmine/wiki/Rest_api#Attaching-files