I've seen a few alerts for the phabricator phd process. It always recovers immediately but there might be something going on with the queue processing. I'll look into it.

Unfortunately I didn't find any useful clues in the logs.

Same thing happened with 1,2 MB files, I think the problem is with thumbnails of files of certain size and up

https://phabricator.wikimedia.org/T280497#7358938

Hmm, perhaps it's hitting a memory limit or something? The limit shouldn't be a problem with such a small file though (unless it's doing something incredibly inefficient.)

My theory is that this has to do with image dimensions ratio. On F34642749, Dimensions says 12,600px × 1,600px (1 ∶ 7.875) while the Default Alt Text incorrectly says (1×12 px, 888 KB).

Default thumbnail mode sets 220px width; there is the sentence "The other dimension of the transform is scaled appropriately, except that we never generate images with crazily extreme aspect ratios." which might apply here.