Google Drive’s 502 error page.
It’s nicely done. Gives the technical information - the error code - and points out it’s an error code for the non-technical users, but not in a patronising way.
Brief explanation with not much detail, but at least keeps it simple. And you don’t need to understand it, because it gives you an action: try again soon.
The image doesn’t get in the way, but lightens it up a bit and makes the page less stark.
Also important: keeping the logo on here. It’d be even better if it said Drive, but eh. This error comes up after a long loading time, so users have probably clicked away from this tab, and forgotten what the page was in the first place.