I had a failure with Gutenberg when saving draft/publishing the first time on an Atomic site, not sure if it's due to the 5 second Jetpack client timeout.
Only able to catch screenshots, as I set up to record it decided to behave and consistently behave. I'll share the screenshots for what happened though. Refreshed a couple times as well and no errors showed up.

This first one is especially weird with the error I received when trying to upload.

Decided to give it a go again with the AT site at Pressable and there seems to be a problem happening again. I was able to catch a gif with the console log(if that helps) this time
Also using Gutenberg Version 4.0.0.
I was also able to post normally using the classic Calypso editor so it's weird what was happening there.
Note that everything works fine now, with no change on my end but creating this for visibility.
cc @Automattic/lannister
I'll keep checking in on it and will update if I can reproduce consistently.
I think this is a timeout problem and will be resolved when D19979-code is deployed.
https://github.com/Automattic/wp-calypso/issues/27822#issuecomment-433569175
D19979-code has been deployed, so I think this problem has been "fixed".
Fixed in D19979 deployment
Thanks @mdawaffe! I didn't realize we had such a long timeout on our v1 APIs.