To clarify, this happened on these very forums.

I have a habit of splitting my screen up into halves, so I can do productive things on one side, while watching videos on another or a chat, but this causes NodeBB's editor to go into 100% mode on me once it hits a certain breakpoint and I like being able to glance back at posts while I'm composing a reply.

This might be one of those Schrodinbugs, but I got an Unexpected end of JSON input while trying to upload an avatar.
This was after trying a few times, and getting rebuffed for having really big avatars. And finally, I found one which wasn't gigantic and it went up.

After that, I decided that I didn't like that avatar and swapped it for another one, and I got that strange error, but the avatar went through, from the looks of it