When uploading posts, sometimes the post gets "stuck" on the processing page (looks like this) if the post is submitted too quickly before the upload finishes preprocessing; despite the page saying "This upload is being processed. Please wait a few seconds", very often the post ends up not fully processing at all. This resulted in the upload not going through the first time. This does not happen if the post is allowed to finish preprocessing.
I'm not sure if this is an error, because in the past, if the post is submitted before the preprocessing is finished, the page will remain on the New Upload page before loading the post page.