Saw this mentioned on Reddit...
UA2494, EWR to IAD on a 757-200. Just over an hour in the air, at just over 6,000 feet basically the entire way (they did get up to 8,000 at one point, but only for a minute or so before dropping back to 6,000). Apparently due to ATC being short-staffed.
https://www.flightaware.com/live/fli.../KIAD/tracklog