This happens when uploading audio with high-quality (i.e. large) files. Every so often they’re too large and the upload will fail. When this happens, there is just a large x in a red circle, but no other information. If I’m uploading just one file, this isn’t a big deal. But if I’m uploading a bunch of recordings from one session, it’s a big bother to figure out which ones failed.
Just to clarify, you’re talking about https://www.inaturalist.org/observations/upload, right? Not one of the mobile apps?
Since it’s audio files they referenced, I assume so, and updated the topic to reflect web. Same issue with very large image files.